Colloidal precursor

Dubau L, Coutanceau C, Gamier E, Leger JM, Lamy C. 2003a. Electrooxidation of methanol at platinum-mthenium catalysts prepared from colloidal precursors Atomic composition and temperature effects. J Appl Electrochem 33 419-429. 

Keywords colloidal precursors, nanosized particles, NMR spectroscopy... 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

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]. 

A preparation and characterization of new PtRu alloy colloids that are suitable as precursors for fuel-cell catalysts have been reported [43cj. This new method uses an organometallic compound both for reduction and as colloid stabilizer leading to a Pt/Ru colloid with lipophilic surfactant stabilizers that can easily be modified to demonstrate hydrophilic properties. The surfactant shell is removed prior to electrochemical measurements by reactive annealing in O2 and H2. This colloid was found to have nearly identical electrocatalytic activity to several other recently developed Pt/Ru colloids as well as commercially available Pt/Ru catalysts. This demonstrates the potential for the development of colloid precursors for bimetallic catalysts especially when considering the ease of manipulating the alloy composition when using these methods. 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

The aim of ceramic membrane production is to obtain defect-free (no cracks, no pinholes) supported films with homogeneous thickness and a narrow pore size distribution. By far the most important are sol-gel based processing routes. They have in common the fact that a porous support is contacted with a colloidal precursor solution for a given time to form a film which is processed after this step. The general procedure can be broken down into the following steps ... 

The process starts with contacting a porous support with the colloidal precursor solution in a dip-coating or a spin-coating process. Film formation starts either with a film-coating or a slip-casting mechanism as will be discussed below. 

The synthesis of mesoporous silica films typically begins with the preparation of precursor solutions. These solutions contain a silica source (typically an alkoxide, although chloride and colloidal precursors can be used), a surfactant molecule used to template the mesostructure, an acid or base catalyst, and solvents. The nanoscale structure is then formed by a cooperative self-assembly of monomeric or partially... 

After the absorption of the dissolved colloidal precursor on solid supports such as... 

Conclusions All three 30wt% PtsoRuso/Vulcan XC 72 electrocatalysts (Catl-3) generated by the respective reduction agents via the colloidal precursor route showed good electrocatalytic activity in methanol oxidation. Small differences in the surface structures, which are a direct consequence of the synthetic methodology used, is likely the reason for the differences in their electrochemical properties. 

The nuclear properties of the nuclei usually found in colloidal oxides and/or hydroxides probed by NMR are shown in Table 1. NMR and H NMR are often instructive of structural modifications in solution or colloidal precursors containing organic ligands, such as the metal alkoxides during their hydrolysis in sol-gel processes, as we briefly discuss in Sec. III. The properties of these two nuclei are very well known [18] and are not discussed here. The two most investigated nuclei... 

The choice of polymer is determined by considering the solubility of the metal colloid precursor, the solvent of choice, and the ability of the polymer to stabilize the reduced metal particles in the colloidal state. Natural polymers such as gelatin and agar were often used before the advent of synthetic polymer chemistry, and related stabilizers such as cellulose acetate, cellulose nitrate [23] and cyclo-dextrins [24] have been used more recently. Thiele [25] proposed the Protective Value as a measure of the ability of a polymer to stabilize colloidal metal. It was defined, similar to the older Gold Number of Zsigmondy, as the weight of the... 

Figure 4.14. Arrhenius plot for methanol electrooxidation at 0.5 V vs. RHE using colloidal PtRu catalyst supported on Vulcan XC72. Electrolyte 1 M CH3OH - 0.5 M H2SO4. Scan rate 1 mV s . Pt Ru atomic ratios 2.33 1, , o 4 1 and Pt/C [96]. (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Elecfrooxidation of methanol at platinum-ruthenium catalysts prepared from colloidal precursors atomic composition and temperature effects, 33, 2003, 419-49, Dubau L, Coutanceau C, Gamier E, Leger J-M, Lamy C, figure 11.)...

In Pt-free electrocatalysts, the surfaces of PdAu/C electrocatalysts are less strongly poisoned by CO than those of PtRu at temperatures of 60 °C. In 2001, Schmidt et al. [65] reported the CO tolerance of PdAu/C (Vulcan XC72) that was prepared via bimetallic colloidal precursors. This work was based on an earlier study by Fishman [375] in which PdAu-black alloys provided a highly active medium for the hydrogen oxidation reaction and a second metal (Au) produced... 

Duhau. L., Coutanceau. C., Gamier, E., etal. (2003). Electrooxidation of Methanol at Platinum-Ruthenium Catalysts Prepared from Colloidal Precursors Atomic Composition and Temperature Effects, J. Appl. Electrochem., 33, pp. 419-429. 

Powders from Colloidal Sol-Based Compotdtions. A common example of the synthesis of a single oxide from a colloidal precursor, following a well-known method (Yoldas, 1975), involves hydrolysis of aluminum secondary butoxide by addition of excess water, formation of boehmite, AlOOH and peptization of boehmite by an add to obtain a colloidal sol. Oh et al. (1996), to dte an application, prepared such colloidal sols and also seeded a part ofsuch a sol with 15 wt% ofa-alumina(<0.12/im). Up to 350°C, both the solid products were amorphous. However, seeding caused early (950°C) crystallization of the a-phase, while the unseeded conqiosition led to a late crystallization (1150°C) of the same phase. 

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