Nanoscale particle preparation

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

C.W. Scheeren, G. Machado, J. Dupont, PEP. Eichtner, S.R. Texeira, Nanoscale Pt(0) particles prepared in imidazolium room temperature ionic liquids synthesis from an organometal-Uc precursor, characterization and catalytic properties in hydrogenation reactions, Inorg. Chem., 42(15), 2003,4738 742... 

Metallic oxides having different shapes and sizes have received considerable attention because of their theoretical, technological applications in various organic reactions. Catalysts, for example, are mostly nanoscale particles, and catalysis is a nanoscale phenomenon. In the case of various reactions, separation of the catalyst from the reaction mixture is the main problem and loss of product occurs. Nano-particles prepared on the resin can easily be applied as a heterogeneous catalyst for efficient recovery and recycling of the photocatalyst from liquid-phase reactions. 

Metallic particles can be prepared by reducing salts dissolved in the droplets. For example, copper particles can be produced using hydrazine as a reducing agent (Pileni, 1993). Composite particles with a core of one metal surrounded by another have also been reported (see Belloni, 1996). Preparation of nanoparticles of various metals has been reviewed by Capek (2004). In some cases, supercritical carbon dioxide (COj) has been used instead of a hydrocarbon as the continuous phase in the microemulsion, which provides a system where the phase behavior can be relatively easily controlled by changing pressure and eliminates the use of volatile solvents. There is considerable interest in making nanoscale particles of semiconductors such as cadmium sulfide (CdS) using water-in-oil microemulsions because their electronic properties are different from those of bulk crystals (see Pileni, 1993). 

The nanoscale particles and related species prepared in microemulsions can be used as precursors for more complex nanostructures and nanomaterials. For example, Pileni and coworkers used Ag and capped Ag2S nanoparticles of narrow size distribution for self-assembly into two-dimensional monolayers and three-dimensional superstructures (153,154,161,163,197-201). 

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