Metals, colloidal preparation

The theories that have been advanced to the effect that the decomposition products of the antiknock dopes and not the compounds themselves are the effective centers of the action has been tested by the use of metallic colloids, prepared in various ways, in the fuels by which engines were operated. The work on colloidal metal sols has been based on the theory that knocking is due to the spontaneous ignition of the unburned charge in an engine cylinder. By acting as catalysts for combustion these substances insure a slow, homogeneous combustion rather than a detonation. 

Tabu- XXIX.—.Uiti-kii>vk Properties of Metal Colloids Prepared at... 

The metal colloid preparations arc mostly based on reduction of metal ions in the presence of bulky anions (e.g. citrate and mercaptoethanesulfonatc), but also quaternary ammonium salts (e.g. tetraoctylammonium bromide) or polymers (e.g. poly(iV-vinylpyr-rolidone (PVP)) are used as stabilizers (Figure 7.2). Ionic species involved in the NP synthesis play a dual role of stabilizers for the metal particles and dopants for the polymer material. In some cases the synthesis of the metal NPs occurs under y-irradiation in the presence of the monomer, which stabilizes the metal particles [64—66,68]. A two-phase approach for the preparation of the monomer or eventually oUgomer-stabUized metal particles has been also suggested in the case of octylthiophen [57] and 3,4-ethylenediox-ythiophene (EDOT) [70,71]. 

The synthetic methods used have induded modem versions of established methods of metal colloid preparation, such as the mild chemical reduction of solutions of transition metal salts and complexes. Some of these reactions have... 

The synthetic methods which have been used include modern versions of established methods of metal colloid preparation such as the mild chemical reduction of solutions of transition metal salts and complexes and newer methods such as radiolysis and photochemical reduction, metal atom extrusion from labile organometallics. And the use of metal vapor synthesis techniques. Some of these reactions have been in use for many years, and some are the results of research stimulated by the current resurgence in metal colloid chemistry. The list of preparative methods is being extended daily, and, as examples of these methods are described below, the reader will quickly be made aware that almost any organometallic reaction or physical process which results in the deposition of a metal is in fact a resource for the metal colloid chemist. The acquisition of new methods requires only the opportunism of the synthetic chemist in turning a previously negative result into a synthetic possibility. 

A gel is defined as a hydrous metal aluminosihcate prepared from either aqueous solutions, reactive soflds, colloidal sols, or reactive aluminosihcates such as the residue stmcture of metakaolin and glasses. 

In summary, we have described our approach towards the synthesis of novel nano-objects consisting of a metal core and a surface that may be functionalized by addition of organic ligands. TEM pictures of the metal core of these nanoparticles appear similar to those of particles commonly used in heterogeneous catalysis or to colloids prepared by well-known reduction methods. However, the organometallic approach displays several specificities which can be summarized as follows ... 

Turkevich who established the first reproducible standard procedure for the preparation of metal colloids [44] also proposed a mechanism for the stepwise formation of nanoclusters based on nucleation, growth, and agglomeration [45,46]. This model, refined by data from modern analydical techniques and results from thermodynamic and kinetic studies, is in essence stiU valid today (Figure 2) [82]. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Reetz et al. have used N-(octyl)4Br-stabilized Pd colloids (typical size, e.g., 3nm) as precursors to form so-called cortex-catalysts, where the active metal forms an extremely fine shell of less than lOnm on the supports (e.g., AI2O3). Within the first 1-4 s, the impregnation of AI2O3 pellets by dispersed nanostructured metal colloids leads to the time-dependent penetration of the support which is complete after 10 s. Cortex catalysts were reported to show a threefold higher activity in olefin hydrogenation than conventionally prepared catalysts of the same metal loading (5% Pd on AI2O3) [388]. 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

In the chemical preparation of unprotected metal colloids, the metal concentration usually has a significant influence on the particle size of obtained metal nanoclusters. For example, when increasing Pd concentration from 0.1 to 1.0 mM in the preparation of Pd metal colloids by the thermal decomposition of Pd acetate in methyl isobutyl ketone, the average Pd particle size increased from 8 to 140nm [6,7]. However, in the alkaline EG synthesis method, the size of metal nanoclusters was only slightly dependent on the metal concentration of the colloidal solution. The colloidal Pt particles prepared with a metal concentration of 3.7 g/1 had an average diameter of... 

Mao and Mao invented a method for synthesizing supported metal catalysts with small metal nanoparticles (1-3 nm) even at high metal loadings (30-50 wt.%) [25]. The obtained metal catalysts exhibited superior electrocatalytic performance in fuel cells. In this invention, the unprotected metal nanocluster colloids prepared according... 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

Protective Colloids. Another approach in preparing and stabilizing metal colloids is by adsorption of macromolecules on their surfaces. A wide variety of materials have been used including gummy gelatinous liquids,(J 0) albumin,(27) Icelandic moss,(28) latex,(22) polyvinylpyrrolidone, (29) antibodies, ( 30 ) carbowax 20M, ( 31 ) polyvinylpyridine, (31 ) and various polymer-water/oil-water mixtures.( 2) These studies clearly indicate that "steric stabilization of metal colloids is also important (along with electronic stabilization).(33)... 

More direct and successful methods for the preparation of non-aqueous metal sols are desirable. Especially valuable would be a method that avoids the metal salt reduction step (and thus avoids contamination by other reagents), avoids electrical discharge methods which decompose organic solvents, and avoids macromolecule stabilization. Such a method would provide pure, non-aqueous metal colloids and should make efficient use of precious metals employed. Such colloids would be valuable technologically in many ways. They would also be valuable to study so that more could be learned about particle stabilization mechanisms in non-aqueous media, of which little is known at the present time. 

Sakka S., Kozuka H., Sol-gel preparation of coating fdms containing noble metal colloids, J. Sol-Gel Sci. Technol. 1998 13 701-705. 

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