Metals, liquid dispersion

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

Figure 16.10 Schematic of a continuous-flow microwave reactor consisting of (a) a liquid column as a pressure regulator, (b) a metal salt solution container, (c) a microwave oven cavity, (d) a metal cluster dispersion receiver and (e) a spiral tube reactor (from [139]).

Chemical nature Metallic compound dispersion Appearance Cream colored liquid... 

Below the mp of the hydride, the metal-gas contact surface, covered with an impermeable layer of hydride, must be renewed constantly for complete hydrogenation. This is done through agitation in the presence of a solid or liquid dispersant. 

In the presence of TMS-Cl the enediolate dianion and, importantly, the alkoxide ions, are trapped as their neutral silyl ethers (Scheme 5). This leads to much improved yields of the coupled product the acyloin is isolated in the form of its silyl enediol ether (3). Work-up is much easier. It is only necessary to filter the solution, evaporate the solvent, and isolate the product by distillation or chromatography. The TMS-Cl should be purified by distillation from calcium hydride, under a nitrogen or argon atmosphere, before use. A convenient procedure when using an organic solvent is to add the ester and the TMS-Cl together, dropwise, to the alkali metal finely dispersed in the solvent, at a rate sufficient to maintain the reaction. An explosion has been reported where this procedure was not followed. For a reaction conducted in liquid ammonia the TMS-Cl is added at the end of the reaction and after all the ammonia has been allowed to evaporate. Particularly in cases where sodium-potassium alloy has been used, a pyrophoric residue may have formed, so that the filtration must be carried out under an inert atmosphere. 

The analysis of hydrogen distribution within the volume of liquid metal, containing dispersed particles of A1203, shows the possibility of precipitation of molecular hydrogen on the oxide particles. 

Examples of liquid-in-gas dispersions are the mist, the clouds, and other aerosols. Liquid-in-liquid dispersions are the emulsions. At room temperature there are only four types of mutually immiscible liquids water, hydrocarbon oils, fluorocarbon oils, and liquid metals — Mercury (Hg) and gallium (Ga). Many raw materials and products in food and petroleum industries exist in the form of oil in water or water in oil emulsions. The soil and some biological tissues can be considered liquid-in-solid dispersions. 

Smoke, dust, and some other aerosols are examples of solid-in-gas dispersions. The solid-in-liquid dispersions are termed suspensions or sols. The pastes and some glues are highly concentrated suspensions. The gels represent bicontinuous structures of solid and liquid. The pastes and some glues are highly concentrated suspensions. Solid-in-solid dispersions are some metal alloys, many kinds of rocks, some colored glasses, etc. 

As shown in Fig. 9, a similar experiment starting with the salen ligand and combining metal insertion and catalyst activation led to the same completion time as the previous experiment. This result demonstrates that the metal insertion step does not limit the overall rate of reaction. The similarities in reaction profile using either pure oxygen (non-scalable) or air at different stirrer speeds confirm the important role of the degree of mixing and gas-liquid dispersion in the activation reaction. Since the overall rate of the complexation and activation sequence is... 

Many disperse systems with solid continuous phase are the common subjects for studies in such areas of science as material science, physics of materials, physics of metals and others. This is related to the existing great variety of such systems. Obviously, their properties (among which mechanical ones are of primary importance) are significantly different from those of systems with liquid dispersion medium. At the same time, the investigation of processes leading to the formation of such systems and their interactions with ambient media constitute direct subjects of colloid science. 

First, let us consider thin-film systems such as emulsions at interfaces. An emulsion is a quasi-stable suspension of fine drops of one liquid dispersed in another liquid. Emulsions, together with microemulsions, can be found in technology, and in almost every part of the petroleum production and recovery process in reservoirs, produced at wellheads, in many parts of the refining process, and in transportation pipelines [1-4]. Understanding the chemistry involved in the stabilization of emulsions and in crude oil emulsions in particular is important both for economic and environmental reasons. The presence of water in oil (w/o) and oil in water (o/w) results in several costly byproducts, such as corrosion, scale, and dissolved metals. Water-in-crude oil emulsions are responsible for the enormous increase in the viscosity of the crude oils produced in reservoirs. Transportation of the viscous crude oil through pipelines is difficult and adds to the cost of production of the oil. With increasing... 

EC-NMR has made considerable progress during the past few years. It is now possible to investigate in detail metal-liquid interfaces under potential control, to deduce electronic properties of electrodes (platinum) and of adsorbates (CO), and to study the surface diffusion of adsorbates. The method can also provide information on the dispersion of commercial carbon-supported platinum fuel cell electrocatalysts and on electrochem-ically generated sintering effects. Such progress has opened up many new research opportunities since we are now in the position to harness the wealth of electronic, Sp-LDOS as well as dynamic and thermodynamic information that can be obtained from NMR experiments. As such, it is to be expected that EC-NMR will continue to thrive and may eventually become a major characterization technique in the field of interfacial electrochemistry. 

Transition metal nanoparticles dispersed in ionic liquids are active catalysts for various reactions, such as the hydrogenation of alkenes, arenes or ketones [102]. Further exanples of ionic liquid-based catalysis with metal nanoparticles are described in the reviews by Gu [78] and Dupont [101]. 

---