Metal sols

Finally, calibration of an unknown scattering pattern is carried out by (1) reducing the intensity in the same way as was done with the scattering of the noble metal sol, (2) obtaining the absolute intensity by... 

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. 

Graham coined the term "colloid" to describe suspensions of small particles in a liquid.(J ) Such particles are generally considered to be from 1 to 5000 nm in diameter and are not easily precipitated, filtered, or observed by ordinary optical microscopes. The topic of this paper is metallic colloidal particles, often called metal sols, with special emphasis on non-aqueous media. Some history of gold sols is appropriate. 

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. 

This value is considerably larger than those reported earlier for aqueous metal sols (43) colloidal gold - 58 mv, 32 mv platinum 44 mv, 30 mv lead - 18 mv. However, such large values might be expected for a low dielectric medium such as acetone. 

F. Porta, L. Prati, M. Rossi, S. Colluccia, and G. Marta, Metal sols as a useful tool for heterogeneous gold catalyst preparation reinvestigation of aUquid phase oxidation, Catal. Today 61(1 ), 165-172 (2000). 

Interaction of Molecules with Particle Surface. We could not explain the observed stability of metal sols in alcohol over several months within the traditional... 

Metal particles. Chemical preparation of metal colloids was initiated by Michael Faraday long ago. A number of procedures have been employed for the preparation of metal sols. In general, the preparation involves the treatment of a metal salt solution with a suitable reducing agent (e.g., NaBH, hydroxylamine) in the... 

Sheflite. Brit WWII expl, same as US 70/30 Tridite (qv), contg 70% PA and 30% 2,6-Dinitro-phenol (DNPh). It was used to cast load AP (armor piercing) bombs and shells. Its properties were yellow cream colored solid d 1.62g/cc mp 83° ballistic strength, 91% TNT brisance, 38.5g sand crushed vs 43.0g for TNT expl temp, 300-15° impact sensitivity, PicArsn app with 2kg wt, 14" vs 14" for TNT rifle bullet impact, 0 to 20% detonations from impact of. 30 cal bullet fired froma90 distance stability, compatibility of metals, sol and toxicity, same as PA (Ref 1)... 

Given the choice of starting materials in hydrothermal reactions, it is worth noting that the larger the spectator ions, the poorer the crystallinity. This is explainable in terms of the electrostatic potential of the metal hydroxides. With lower electrostatic potentials comes a greater likelihood that the metal sol will undergo oleation and cross-link. 

Colloidal iron, together with a small quantity of ferrous hydroxide and a ferrous salt, is obtained by electrolysing a solution of pure gelatin and a dilute solution of an alkali chloride or sulphate between iron electrodes.2 The sol resembles those of lead and bismuth in that it bears a positive charge. The majority of metallic sols, on the other hand, are negative. 

Mie, G. (1908). Considerations on the Optics of Turbid Media, Especially Colloidal Metal Sols./4 . Physik 25 377-445. 

Mixing a preformed metal sol with a support material also provides a method for the preparation of supported catalysts with the colloidal metal particles attached to supports such as alumina - o, titania5. 52 d pumice.53 While this procedure gives catalysts having essentially a single size metal particle, the particles are not strongly bonded to the support which makes these materials primarily useful for vapor phase reactions. An added complication is that the citric acid commonly used to prepare the sols > >52 qj. the micellar material in which they are stabilized, can also be adsorbed on the support and, possibly, inhibit the activity of the resulting catalysts. 

The electronic absorption spectrum of metal nanocrystals in the visible region is dominated by the plasmon band. This absorption is due to the collective excitation of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being well-known [6-8]. The dependence of the plasmon peak on the dielectric constant of the surrounding medium and the diameter of the nanocrystal was predicted theoretically by Mie and others at the turn of the last century [9-12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7]. 

Deposition. - Deposition, as used in preparing supported catalysts, is the laying down or placing of the active components on the exterior surface of a support. One means by which this may be achieved is the preparation of catalysts by sputtering, which involves condensing the metal vapour onto an agitated finely dispersed support. However, as this process is performed under a high vacuum, the technique is probably only useful for the preparation of model catalysts. Alternatively, the process may be performed in the liquid phase by the deposition of a metal sol onto a suspended support. 

An electrical double layer arises at the semiconductor elec-trode/electrolyte solution interface, as in the case of the metal/sol-ution interface. The double layer consists of the plates carrying charges of opposite sign, each plate being located in one of the phases in contact. In the near-surface region of the semiconductor the charge is formed as a result of redistribution of electrons and holes, while in the solution it is formed as a result of ion redistribution. Under equilibrium conditions, the absolute values of these charges are the same. 

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. 

The results of Olin and Jebens 110 shown in Table XXIX do not harmonize with the results of either of the two preceding sets of experiments. A successive decrease in useful compression ratio is shown by successive increases in temperature of decomposition of the organo-metallic compound. The conditions for maintaining stability of the metallic sols were comparable to those of Sims and Mardles. Hence, it appears that Sims and Mardles may have erred in not obtaining as complete a decomposition of tire organir compounds as would be necessary to show effectively the action of the colloidal metals. 

A somewhat similar theory10 postulates the formation of colloidal lead by the decomposition of lead tetraethyl, which deposits on sharp points, edges, and projections in the cylinder which would otherwise aid reaction to an extent that a detonation wave would result. The theory fails to explain the action of organic amines, of di- and tetravalent. selenium, of the colloidal metal sols, and fails to account for the immediate recurrence of knocking when the antiknock dope is discontinued in the gasoline feed. However, tubes coated with lead oxide have been found to reduce the oxidation of hexane more than lead tetraethyl vapors.1158 On the basis of this finding the hypothesis was advanced that oxidation occurs to some extent prior to compression and at the first contact of the gaseous mixture with the hot walls, aud that in the presence of the lead... 

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