Particles from Microemulsions

Introduction. - During the past few years, a number of papers dealing with the preparation of metallic particles from microemulsions have appeared. For example Pd, Ni, Pt, Ag and bimetallic Au/Pd were successfully prepared by this method . For this reason, it is of interest to report the research work done on this subject and its input on catalysis research. 

One of the most important conditions for obtaining monodisperse colloidal particles from microemulsions is shown to be the differentiation between nucleation and growth. Detailed analysis showed that the formation of Ni2B or C02B particles was governed by the initial composition of the microemulsion. In most cases the particles obtained are larger than... 

PARTICLES FROM MICROEMULSIONS 5.4.1. Single and Multiple Oxides... 

Boutonnet, M., Kizling, J., Stenius, P., and Maire, G. 1982. The preparation of monodisperse colloidal metal particles from microemulsions. Colloids and Surfaces, 5, 209-225. 

M. Boutonnet, J. Kizling, P. Stenius, and G. Maire, The preparation of monodi ierse colloidal metal particles from microemulsions. Colloids Sutf. 5,209-225 (1982). 

Chang CL, Fogler HS (1997) Controlled formation of silica particles from tetraethyl orthosUicate in nonionic water-in-oil microemulsions. Langmuir 13 3295-3307... 

When we consider the metals of nanoscopic size, fine metal particles from micrometer to nanometer size can be synthesized by both physical and chemical methods. The former method provides the fine metal particles by decreasing the size by addition of energy to the bulk metal, while in the latter methods, fine particles can be produced by increasing the size from metal atoms obtained by reduction of metal ions in solution. Since chemical reactions usually take place in homogeneous solution in any case, this chapter includes most of the cases of synthesis and growth of fine metal particles. However, the polyol process, reaction in microemulsions, and formation in the gas phase are omitted, since they are described in later chapters by specialists in those fields. 

The aim of the present investigation is to synthesize nano-sized particles from CaCC>3 and BaCC>3 by the microemulsion method. 

The results sustained the opinion that the synthesis of nano-particles in microemulsion (w/o) is to be preferred. In common emulsions, both phases spontaneously separate from each other while microemulsions are thermodynamically stable, do not segregate and appear to be transparent. This can be explained with the size of the water droplets [14,11]. Microemulsions are also characterized by the so called dynamic exchange process . The emulsion droplets in such emulsions constantly integrate and disintegrate, thus exchanging substance between each other. 

In some other cases, the noble metals were deposited onto ceria from colloidal suspensions. Tlius, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(i%)/Ce02 catalyst (182). Likewise, a series of Pd/Ce02 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofiiran (78). 

Fig. 4 Histograms of platinum particles determined by TEM in the catalysts prepared from microemulsions (0.1 % Pt)...

Ganguli, D. and Ganguli, M. (2003) Particle formation from microemulsions A survey. In M.P. Pileni (ed), Inorganic Particle Synthesis via Macro- and Microemulsions. A Micrometer to Nanometer Landscape. Kluwer Academic/Plenum Pubhshers, New York, p. 97. 

Espiard, P Guyot, A Mark, J Surface functionalized colloidal silica particles from an inverse microemulsion sol gel process. J. inorganic and organometallic polymers 1995, 5, 391-407. 

Figure 3 Procedure of metal particle preparation from microemulsion. The obtained Pd particles displayed an average size of 7 nm...

In this section we will focus on the description of several aspects relevant to the preparation of both catalytically active metal particles and metal-supported catalysts via the microemulsion technique. Regarding the metal supported catalysts, in some cases both the metallic particles and the support were synthesized by microemulsions. However, in general metallic particles prepared from microemulsions were deposited on commercial supports. The catalytic behaviour of these microemulsion-derived materials will be commented and, when possible, compared to catalysts obtained from traditional techniques under similar reaction conditions. Selected results concerning the study of the strong metal-support interaction effect (SMSI) obtained with catalysts prepared by microemulsion will be detailed . Several papers dealing with the preparation of immobilized metal particles on supports have been described although the catalytic behaviour of the solids was not studied. However, their potential catalytic ability led us to include those papers within this chapter. 

Unsupported Catalysts. - Most of the research work is focused on the preparation of noble metal catalysts using microemulsions in the metal particle synthesis. It is now well known that the early work of Boutonnet et al opened a new and simple way to preparing stable reduced noble metal particles from their salt precursors. In their work, metallic particles of platinum, rhodium, palladium and iridium were obtained from their salt precursors, HaPtCl, RhCh, PdCh and IrCh, respectively, in water in oil (w/o) microemulsions. For Pt and Pd, the most suitable reducing agent was hydrazine. In the case of Rh " and Ir the... 

The methodology was first reported by Boutonnet et al.. They obtained a supported metal catalyst by depositing ultra-fine particles synthesized in a w/o microemulsion on pumice. The activity of this catalyst was tested in the but-1-ene hydrogenation reaction and compared with the activity of similar catalysts prepared by the traditional impregnation method both from aqueous and alcoholic solutions. The particle size was found to be in the 20-30 A range for the particles prepared from microemulsions and above 200 A for the classical impregnation counterparts. The behaviour of the Pt-based catalyst was found to depend on the preparation method. However, such a correlation between the activity and the preparation method could not be established in the case of Rh and Pd-based catalysts. 

Liu et al. have prepared PtRu/C catalysts from microemulsions and emulsions . Their results show how particles prepared from microemulsions displayed a lower particle size than the emulsion counterparts. 

The special properties of the catalysts prepared from microemulsion systems include in addition to a small and uniform particle size also unique features related to the special environment in which particle formation takes place. However, further studies are still required in order to understand the relation between the mode of preparation and the properties of the particles. 

Figure 19 Model of the preparation of colloidal C02B particles from water-in-oil microemulsion.

Fig. 5.10 Barium titanate particle size distribution originating from microemulsion synthesis (a) and HRTEM image of single particle (b)...

Synthesis of CdS-ZnS particles from reverse microemulsions AOT/isooctane/ aqueous solution of Cd- and Zn- nitrates or Na2S and their rapid mixing at 25°C was reported by Hirai and colleagues [356,357]. Though the products were verified to be solid solutions, i.e. Cdj. Zn S, it was also found that the particles in each case contained less zinc than that available in the feed solution. This was explained to be due to the lower solubility product of bulk CdS than that of ZnS, and so, higher availability of Zn in the solution. However, with increasing conversion of the metal ions to sulfides, the composition progressively approached that of the... 

Ward etal [386] reported synthesis of quantum size PbS particles from a reverse micellar system constituted of dodecyl tetraethoxy ethylene glycol ether (mixture of Neodol 23-3 and 23-7, designated as C,2(EO)4) and hexane. Reverse microemulsions were obtained by addition of aqueous solutions of Pb-nitrate/perchlorate or Na2S. Synthesis was accomplished by either mixing of the two microemulsions or addition of sodium sulfide solution into the Pb-containing system. The particle size varied in the range 1-10 nm with the median size of 3 nm. 

Platinum/palladium nanoparticulate catalysts were synthesized [421] from microemulsions in the system PEGDE/hexadecane/water. The conversion of the platinum precursor to metal nanoparticles (10-100 nm, formed by aggregation of 2-5 nm particles) was achieved through hydrazine at ambient temperature. The nanoparticles could be deposited on alumina substrates. 

Iridium particles were obtained [242, 243] from microemulsions in the system PEGDE / hexane / aqueous solution of IrCl. The reduction was achieved through hydrogen. The particles had an average diameter of 2.5 nm. 

A major application of oxide nanoparticles is fabrication of ceramics (including substrates) with dense microstructure. Synthesizing such particles from macroemulsions (as compared to microemulsions) adds the advantage of using only small quantities of surfactants in the formulations and keeping the system low in pollutants. Examples of such endeavors are discussed below. 

While effective removal of surfactant molecules from freshly precipitated nanoparticles constitutes one of the involved issues in microemulsion synthesis, a coating of surfactants on nanoparticles has been found to improve their luminescence behavior in certain cases. Thus, Fei etaL [284] synthesized a-Fe203 particles from reverse microemulsions in which sodium dodecyl benze sulfonate (DBS) was the selected surfactant. The a-Fe203 organosols with the particles coated by DBS and heat-treated at 140 C showed strong visible luminescence while the uncoated particles prepared under similar conditions were not luminescent. 

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