Metals, colloidal effect

Solvents such as organic liquids can act as stabilizers [204] for metal colloids, and in case of gold it was even reported that the donor properties of the medium determine the sign and the strength of the induced charge [205]. Also, in case of colloidal metal suspensions even in less polar solvents electrostatic stabilization effects have been assumed to arise from the donor properties of the respective liquid. Most common solvent stabilizations have been achieved with THF or propylenecarbonate. For example, smallsized clusters of zerovalent early transition metals Ti, Zr, V, Nb, and Mn have been stabilized by THF after [BEt3H ] reduction of the pre-formed THF adducts (Equation (6)) [54,55,59,206]. Table 1 summarizes the results. 

Contrary to previous reports suggesting colloidal metal as the active species in Pt-catalyzed hydrosilylations, the catalyst was found to be a monomeric platinum compound with silicon and carbon in the first coordination sphere.615 The platinum end product at excess olefin concentration contains only platinum-carbon bonds, whereas at high hydrosilane concentration, it is multinuclear and also contains platinum-silicon bonds. An explanation of the oxygen effect in hydrosilylation was also given to show that oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. 

Platinum and palladium in colloidal solntiou aro used for effecting the hydrogenation of substances in aqueous or alcoholic solution. Hydrogen is passed through the solutions and it is often advantageous to work under slightly increased pressure the colloidal metals are obtained from palladious clilorido or potassium platinicliloride, in the presence of guiuarabic. 

Comparison of the efficiencies of different colloidal metals may be established by determining the relative volumes of hydrogen used for reduction by them m unit time. This was effected by conducting hydrogen into a flask containing... 

As mentioned above, the stability of colloidal metal solutions is considerably enhanced by the addition of certain organic substances or protective colloids. For example, gelatin has been frequently employed, 0-5 gram per litre of colloidal solution proving very effective in preventing the coagulation or precipitation of the metal.2 Extracts of Iceland moss3 and of quince seed 4 have also been recommended. 

Colloidal metal. A fine suspension of colloidal potassium is obtained by sonication in toluene or xylene under argon at 10°. This material is highly effective for Dieckmann condensation of dicarboxylic esters. 

The sols are easy to prepare (see also Lee and Meisel, 1982) and the molecules whose SERS spectra are to be obtained are added to the sol in very small concentrations (10 ". ..10 mol/1). Recently a new method for preparing SERS active colloids has been pioneered by Neddersen et. al. (1993). By means of laser ablation of metals with a Nd YAG laser operating at 1064 nm, stable Ag, Au, Pt, Pd, and Cu colloids were directly prepared in water and organic solvents. An important advantage of this method over conventional chemical procedures is that the colloids are free of organic or ionic species. Consequently, the chemical and physical effects of ions or other adsorbates can be studied under carefully controlled conditions. The authors report that the SERS activity of colloidal metals prepared by this laser ablation method is comparable or even superior to that of chemically prepared colloids. 

The use of colloidal metallic nanoclusters deposited onto solid substrates can provide a higher degree of control over the SPs spectral properties state-of-the-art results in the chemical synthesis showed, in fact, the possibility to grow nano-objects with high uniformity and low size dispersion [36-38], This can allow to fabricate extended substrates, in which the local morphology, and thus the resulting MEF effect, can be controlled with good precision [19, 39], As a counterpart, still some randomness is unavoidable in this approach, since it is not simple to define the position of the nanoclusters on the substrate with micrometer precision to realize, for instance, ordered arrays of metallic particles. 

Although the connection between SERS and SPPs is established in (1.2), how to effectively produce desirable SERS on a given metallic structure remains challenging. To address this issue, different plasmonic systems at different dimensionalities have been proposed, fabricated, and characterized with the same intention to maximize the local field strength at cOexc and cors [13-16]. A variety of structures have been found to support SERS well, including roughened metal electrodes, metal nanoparticles colloids, metal island films, metal nanorods, etc., or more specifically, substrates that contain nanoscale gaps or hotspots are particularly... 

This basic displacement-alkylation exchange is catalyzed very effectively by nickel. Present studies have been restricted to the use of various nickel compounds and it has been concluded that colloidal, metallic nickel is the most active form. In practice, addition of some easily reduced nickel salt—i.e., nickel formate—to the alkylaluminum mixture results in efficient reduction to colloidal nickel. 

The hydrosilylation reaction can also be conventionally conducted by reaction of an olefin and an SiH-f mctional polydimethylsiloxane in the presence of a standard transition metal catalyst, and after the reaction the catalyst can be extracted with an ionic liquid. In some cases, the use of an ionic liquid in the hydrosilylation process even improved the quality of the polyethersiloxanes with respect to color compared to the standard process. An explanation might be the avoidance of catalyst reduction leading to the formation of colloidal metal particles, which tend to color the product slightly brownish. In other words, the ionic liquid seems to have a stabilizing effect on the catalyst. 

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. 

Table XXVIII.—Effect of Colloidal Metals in Raising the H.U.C.R. of Gasoline.

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. 

Since colloidal particles of silver and gold also give large SERS effects, it is useful to consider these as disconnected microelectrodes. Controlled variations in the potential at the interface between silver sol particles and electrolyte solution have been achieved by adding the Eu3+/Eu2+ redox system and varying the concentration ratio [22]. From measurements of SER spectra of pyridine adsorbed on the silver sol, the potential-dependent effects have been found to be identical with those given by bulk silver electrodes. These experiments also demonstrated that the intensity ratio of the pyridine Raman bands at ca. 1010 and 1040 cm-1 may be used as a simple but effective measure of sin-face potential for colloidal metals. 

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