Metal salts Microemulsion

This method involves formation of reverse micelles in the presence of surfactants at a water-oil interface. A clear homogeneous solution obtained by the addition of another amine or alcohol-based cosurfactant is termed a Microemulsion. To a reverse micelle solution containing a dissolved metal salt, a second reverse micelle solution containing a suitable reducing agent is added reducing the metal cations to metals. The synthesis of oxides from reverse micelles depends on the coprecipitation of one or more metal ions from... 

Among the different possibilities, the water-in-oil microemulsion method [63-73] can be a good approach. This method was derived from that developed by Boutonnet et al. [64], Two different microemulsions are prepared, one containing the metal salts (e.g., 99.9% II2PtCI6, SnCl2,... 

Microemulsions are used as reaction media for a variety of chemical reactions. The aqueous droplets of water-in-oil micro emulsions can be regarded as minireactors for the preparation of nanoparticles of metals and metal salts and particles of the same size as the starting microemulsion droplets can be obtained [1-3]. Polymerisation in micro emulsions is an efficient way to prepare nanolatexes and also to make polymers of very high molecular weight. Both discontinuous and bicontinuous micro emulsions have been used for the purpose [4]. Microemulsions are also of interest as media for enzymatic reactions. Much work has been done with lipase-catalysed reactions and water-in-oil microemulsions have been found suitable for ester synthesis and hydrolysis, as well as for transesterification [5,6]. 

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

Figure 11 illustrates the formation of metallic nanoparticles using reverse microemulsions as an example of growing particles within individual dispersed phases or molecular containers. In this process, a metal salt is solubilized in the aqueous interior of... 

The metal salts are taken in w/o microemulsion in a container. Concentrated solution of the reductant or the desired reacting salt is then injected into the solution to perform the reaction process. The produced nanoparticles of the metals, viz. Cu, Ag, Au, Pd, Pt etc., or their salts can be isolated by destabilisation of the microemulsion system and washing and cleaning the products and storing them in inert conditions as required. 

The metal salts and the reductants or two reacting salts are taken in the water pools of the same microemulsion system. They are then slowly mixed with constant stirring as per stoichiometric requirements. The process of reduction or reaction takes place in situ and the desired nanoclusters of the metals or their desired salts are formed. They can be separated, washed and stored as described above. 

Recently it was proposed that PEMLC electrocatalysts may also be prepared by water-in-oil microemulsions. These are optically transparent, isotropic, and thermodynamically stable dispersions of two nonmiscible liquids. The method of particle preparation consists of mixing two microemulsions carrying appropriate reactants (metal salt + reducing agent), to obtain the desired particles. The reaction takes place during the collision of water droplets, and the size of the particles is controlled by the size of the droplets. Readers are referred to the early work of Boutonnet et al. [149], the review paper of Capek [150] and refs. [128,151], and 152 for fuel cell apphcations. The carbonyl route has the ability to control the stoichiometry between bimetallic nanoparticles, but also the particle size. The reader is referred to review papers for more details [106,107]. Other methods, including sonochemical and radiation-chemical, have been used successfully for the preparation of fuel cell catalysts (see, e.g., review articles 100 and 153). 

In the case of metal particle preparation the choice of the metal precursor is of paramount importance. Obviously, water-soluble precursors are desired, generally transition metal salts, but even then different behaviours may be expected from different precursors. From Table 4 it can be observed that the solubility of chloroplatinic acid in a microemulsion is seven times higher than that of rhodium, iridium and palladium chlorides l... 

Table 4 The solubility of different transition metal salts in a microemulsion of surfactant (Pentaethyleneglycoldodecylether (PEGDE), CnEs), iso-octane and water...

Microemulsion composition Concentration Metal saltfs) Concentration of metal salt in the microemulsion mmole/kg... 

Effect of the metallic precursor. An important factor controlling the final particle size of the catalysts is the metallic precursor. Most papers on the subject report the use of water-soluble metallic salts as precursors. However, this is not always a straightforward option in order to obtain better catalysts. For instance, it was stated for Pt catalysts that a better control of both particle size and particle size distribution could be achieved by the preparation of Pt complexes, nanoparticles, formed in water in oil microemulsions . ... 

Recently, taking advantage of the very narrow size distribution of the metal particles obtained, microemulsion has been used to prepare electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) Catalysts containing 40 % Pt Ru (1 1) and 40% Pt Pd (1 1) on charcoal were prepared by mixing aqueous solutions of chloroplatinic acid, ruthenium chloride and palladium chloride with Berol 050 as surfactant in iso-octane. Reduction of the metal salts was complete after addition of hydrazine. In order to support the particles, the microemulsion was destabilised with tetrahydrofurane in the presence of charcoal. Both isolated particles in the range of 2-5 nm and aggregates of about 20 nm were detected by transmission electron microscopy. The electrochemical performance of membrane electrode assemblies, MEAs, prepared using this catalyst was comparable to that of the MEAs prepared with a commercial catalyst. 

Water-in-C02 microemulsion was used to dissolve metal salts in the production of nanoparticles via RESOLV. In order to evaluate the solubility of Cu(N03)2, for example, the same microemulsion as that used in the rapid expansion was prepared in a high-pressure optical cell. With Cu(N03>2 in the water phase, which exhibited the distinctive blue color of aqueous Cu (70), the microemulsion appeared homogenous. According to the observed absorbance (the band centered at 740 nm), the Cu(N03)2 salt was completely dissolved in the PFPE-NH4-stabilized water-in-C02 microemulsion. The other metal salts were similarly soluble, resulting in microemulsions that appeared equally homogeneous. 

As delineated in the Introduction, numerous research groups have focused on the preparation and application of transition metal nanoparticles in the zerovalent form during the last two decades but, parallel to this development, research in the area of nanosized metal oxides has also increased. The areas of application of metal oxide nanoparticles range from catalysis [4] to semiconductors (e.g., ZnO, ZnS, CdSe) [5]. In the area of heterogeneous catalysis, many different preparative methods have been described [4], Other methods are based on the hydrolysis of transition metal salts in microemulsions [3-8], These and other approaches have been reviewed and will not be specifically treated here. Rather, the main focus is on the author s own research. 

As a solution-based materials synthesis technique, the microemulsion-mediated method [10-18] offers the unique ability to effect particle synthesis and particle stabilization in one step. The solubilized water droplets serve as nanosize test tubes, thus limiting particle growth, while the associated surfactant films adsorb on the growing particles, thereby minimizing particle aggregation. The purpose of this chapter is to review the literature on the microemulsion-mediated synthesis of metal hydroxides and oxides the definition of a metal is extended here to include the semimetal silicon. Since metal oxides are frequently produced by decomposing metal salts, aspects of the literature on microemulsion-derived metal salts are also considered. In principle, any previously established aqueous precipitation chemistry can be adapted to the microemulsion synthesis technique. Accordingly,... 

In order for microemulsion-based materials synthesis to be feasible, surfactant/oil/water formulations that give stable microemulsions must be identified. Phase diagrams already available in the literature [122-124] provide a useful starting point. Frequently, however, these published diagrams do not extend to conditions directly relevant to materials synthesis, e.g., in terms of the specific metal salt, base, acid, and temperature. Of important consideration, therefore, are investigations into the effects of the reactants... 

The susceptibility of microemulsions to destabilization by electrolytes severely limits the highest metal concentrations that can be used for precipitation reactions. This, in turn, discourages the large-scale application of microemulsion-mediated materials synthesis. A possible approach to tackling this problem appears to lie in the judicious selection of cosurfactants for microemulsion formulations. Darab et al. [125] reported that addition of SDS to the AOT/isooctane/water microemulsion increased dramatically the tolerable concentration of metal salts in the water pools. According to Chhabra et al. [50], addition of -hexanol to the Triton X-lOO/cyclohexane/water microemulsion led to a significant improvement in the water-solubilizing capacity. 

Takatori and his collaborators in Toyota Research Center were one of the first who developed and systematically studied the emulsion combustion method (ECM) [3]. This method is basically a combination of the microemulsion wet chemistry and the flame spray pyrolysis methods. In ECM, an aqueous solution of a metal salt is mixed with a fuel such as kerosene and a small amount of an emulsifier or surfactant to obtain a water-in-oil (W/O) type of emulsion. Using a spray nozzle, the solution is then atomized to produce a spray. The size of the mother emulsion droplets depends on the atomizer type and the atomization conditimis, and is on the order of 10 pm for air-assist nozzles. The size of the dispersed micro-solution droplets depends on the string process and surfactant, and is about 1 pm [3]. Figure 40.1 shows a schematic diagram of the ECM. 

The basic model as outlined above (often described as fusion and fission of droplets in W/O microemulsions) has been generally used for explaining reactions leading to, for example, polymerization of monomers and reduction of metal ions to metal particles. Natarajan et al. [160] who proposed a stochastic model for ultrafine metal particle synthesis from metal salts by the above method worked out some fusion-fission rules as summarized below ... 

The method for the preparation of metal nanoparticles within micells consists of forming two microemulsions, one with the metal salt of interest and the other with the reducing agent, and mixing them together. A schematic diagram is shown in Figure 6.6. 

Spinel ferrites can be synthesized in microemulsions and inverse micelles. For example, MnFejO nanoparticles with controllable sizes from about 4-15 nm are synthesized through the formation of water-in-toluene inverse micelles with sodium dodecylbenzenesulfonate (NaDBS) as suifactanP. This synthesis starts with a clear aqueous solution consisting of Mn(N03)2 and Fe(NOj)j. A NaDBS aqueous solution is added to the metal salt solution, subsequent addition of a large volume of toluene forms reverse micelles. The volume ratio of water and toluene determines the size of the resulting MoFCjO nanoparticles. 

Metallie nanoeatalysts ean be incorporated into the mesoporous structure by a variety of methods. Prefabricated nanopartieles can be incorporated into mesoporous solids by adding the partieles into the sol-gel mixture or, if the particles are formed by mieroemulsions (see Section 9.2.5), the microemulsion can be incorporated into the preformed mesoporous structure. Alternatively, metal salts can be added during gel formation or after the mesoporous structure has formed [12]. An example of the former is the synthesis of WO, and WO Pt films that were made by synthesizing W(OC2H5)e and H2PtCl6 sol-gel solutions, followed by aging and calcination [13]. A significant drawback associated with this method is that the catalytic nanoparticles may be buried within the structure rather than near the pores. If the partieles are not located near the pores they will not be accessible to reactants and therefore will not be efficient catalysts. 

Microemulsions with ILs acted as catalytic activity enhancer for oxidases. Zhou et al. [95] reported a water-m-[bmim][PFJ microemulsion system stabilized by TX-lOO that enhances the catalytic activity of hgnin peroxidase (LiP) and laccase. Optimum molar ratios of water to TX-lOO were 8 and >20 for UP and laccase, respectively. Compared to pure or water-saturated [bmim][PFJ, the derived microemulsion evidenced enhanced catalytic activity. Use O/W for oU-in-water ivation effect of [bmim][PFJ on LiP and laccase. Xue et al. [96] reported timable enzyme (laccase) activity in a microemulsion system, water/AOT+TX-100/[bmim][PFJ. The solution of IL [bmim][Cl] and polar organic solvent formamide (FA) were used to form a nonaqueous microemulsion as [bmim][Cl]-FA/TX-100/cyclohexane (Rg. 10.8) at 25 0.1°C, reported by Wei et al. [97]. By means of electrical conductivity, dynamic hght scattering (DLS) and UV-Vis spectroscopy measurements, microstructures, internal phases, and size regime were explored of the aforesaid microemulsion system. UV-Vis studies using CoCl indicated metal salt dissolution by microemulsion. 

Type A copolymers and terpolymers have been prepared by copolymerizing vinyl pyridinium halides with alkali metal salts of sulfonate comonomers including vinylsulfonate, 2-acrylamido-2-methyl propane sulfonate, and p-styrene sulfonate (15-19). Methacrylamidopropyl-trimethyl ammonium chloride and p-styrene sulfonate have been terpolymerized with the hydrophilic monomer acrylamide (20,21). Type A copolymers and terpolymers have also been prepared from microemulsions of sodium 2-acrylamido-2-methyl-l-propanesulfonate and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (22-25). 

To ensure that the experimental procedure adopted results in the highest-possible colloidal nanoparticle concentration, the sequence of precursor addition was reversed. A second scheme which involved mixing the metal salt powder with microemulsions already containing the stoichiometric amount of NaOH solution was tested. Both schemes succeeded in forming stabilized colloidal nanoparticles however, higher uptake was obtained when scheme 1, solubUizing the metal salt before adding NaOH, was employed. The lower uptake associated with scheme 2 was attributed to the formation of a mass transfer barrier of the metal oxide/hydroxide at the surface of the salt powder, which prevented further dissolution. 

Following the experimental procedure outlined for nonreactive surfactants, it was found that a range of R between 2 and 8, at fixed valnes of other microemulsion variables, snccessfully maintained stable colloidal particles of the different metal oxides. AtR<2, all the water was bounded to the snrfactant heads and no water was available to solubilize the metal salt precursor. For / > 8, on the other hand, cloudiness appeared indicating a shift towards Wmsor type n microemulsions. 

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