Metallic colloidal activity

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Reetz et al. have used N-(octyl)4Br-stabilized Pd colloids (typical size, e.g., 3nm) as precursors to form so-called cortex-catalysts, where the active metal forms an extremely fine shell of less than lOnm on the supports (e.g., AI2O3). Within the first 1-4 s, the impregnation of AI2O3 pellets by dispersed nanostructured metal colloids leads to the time-dependent penetration of the support which is complete after 10 s. Cortex catalysts were reported to show a threefold higher activity in olefin hydrogenation than conventionally prepared catalysts of the same metal loading (5% Pd on AI2O3) [388]. 

SERS-active suspensions of elemental metal colloids or nanoparticles of various sizes can be chemically formed in solution. Silver colloids can easily... 

A. Doron, E. Katz, and I. Willner, Organization of Au colloids as monolayer films onto ITO glass surfaces application of the metal colloid films as base interfaces to construct redox-active monolayers. Langmuir 11, 1313-1317 (1995). 

Finally, the development of modified nanoparticles having better stability and a longer lifetime has involved interesting results in diverse catalytic reactions. Efficient activities are obtained with these transition-metal colloids used as catalysts for the hydrogenation of various unsaturated substrates. Consequently, several recent investigations in total, partial or selective hydrogenation have received significant attention. 

Bonnemann H. and Brijoux, W., in Catalytically Active Metal Powders and Colloids/Active Metals, Ftirstner, A., Ed., VCH, Weinheim, 1996, pp. 339-379. 

A promising strategy towards stable and catalyticaUy active metal colloids is their preparation inside the core of micelles formed by amphiphilic block copolymers. This strategy offers a number of advantages (i) micelles represent a nano-structured environment which can be exactly tailored by block copolymer synthesis (ii) polymers act as effective steric stabilizer ]36] (iii) metal leaching might be avoided (iv) micelles allow control over particle size, size distribution and particle solubility [37] and (v) micelles are also supposed to effect catalytic activity and selectivity [38]. 

Relative catalytic activity of these metal colloids in the hydrosilylation of vinyltrimethylsilane with HSi(CgHi3-n)3 and HSi(OEt)3 (25 ppm of catalyst at ambient temperature) is determined to be in the order Pt > Rh > Ru Ir ) > Os, and Pd-colloid does not show any activity29. The catalytic activity depends on the nature of the hydrosilane used, i.e., Ru, Ir, and Os show almost negligible activity with HSi(CgHi3-n)3, while Ru and Ir display much higher activity, equivalent to Rh, with HSi(OEt)3. 

The process which we have applied successfully for the reduction of uracil is based on the method of reduction developed by Skita.8 This investigator used both colloidal palladium and platinum in his work and utilized as a support for his colloidal metal gum arabic. He showed also that his colloidal metals were active in both acid and alkaline solutions. 

Biffis A (2001) Functionalised microgels novel stabilisers for catalytically active metal colloids. I Mol Catal A Chem 165 303-307... 

In order to investigate molecules adsorbed at the solid-liquid interface roughened electrode surfaces or metal colloids in solution (sols) are prepared. For investigations of the solid-gas or solid-vapour interface several methods are available to produce metal island films on SERS-active substrates. 

The preparation of metal colloids and their use in catalysis is almost as old as the study of catalysis itself and their efficiency as catalysts is well established. In fact the high activity of colloidal metals in a number of catalytic processes has sometimes been a complicating factor in the investigation of... 

The self-assembly technique has attracted much attention since they were observed by Decher in 1991 [49]. Self-assembly is the fundamental principle that provides the precise control of the resulting assemblies and the thickness of an individual layer on the nanometer scale by variation in the bulk concentration of the metal colloids suspension, deposition time, pH, and transport conditions [50]. Recently, the functionalization of metal nanoparticles has opened up new opportunities for the construction of nanostructured self-assembly films to fabricate novel SERS-active Ag substrates. 

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