Metals, colloidal reduction catalysts

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Recently, Liew et al. reported the use of chitosan-stabilized Pt and Pd colloidal particles as catalysts for olefin hydrogenation [51]. The nanocatalysts with a diameter ca. 2 nm were produced from PdCl2 and K2PtCl4 upon reduction with sodium borohydride in the presence of chitosan, a commercial biopolymer, under various molar ratios. These colloids were used for the hydrogenation of oct-1-ene and cyclooctene in methanol at atmospheric pressure and 30 °C. The catalytic activities in term of turnover frequency (TOF mol. product mol. metal-1 h-1)... 

Metal hydrogenation catalysts may be employed in any one of a variety of forms (a) macroscopic forms as wires, foils or granules (b) microscopic forms as powders obtained by chemical reduction, colloidal suspensions, blacks or evaporated metal films (c) supported catalysts where varying concentrations of metal are dispersed to a varying degree on a carrier such as alumina, silica or carbon. 

Since the use of alcohol in the sol-gel process can lead to aggregation of the colloidal metal, Au/SiC>2 catalysts have been prepared without alcoholic solution, using tetramethyl orthosilicate (TMOS) as a water-soluble silicic precursor and colloidal gold by reducing aqueous HAuCLi with magnesium citrate.114 However, the size of the gold particles was not reported, but reduction by citrate ion does not usually produce small particles. 

The hydrosilylation reaction can also be conventionally conducted by reaction of an olefin and an SiH-f mctional polydimethylsiloxane in the presence of a standard transition metal catalyst, and after the reaction the catalyst can be extracted with an ionic liquid. In some cases, the use of an ionic liquid in the hydrosilylation process even improved the quality of the polyethersiloxanes with respect to color compared to the standard process. An explanation might be the avoidance of catalyst reduction leading to the formation of colloidal metal particles, which tend to color the product slightly brownish. In other words, the ionic liquid seems to have a stabilizing effect on the catalyst. 

Research has also focused on understanding the mechanism of the transition metal-catalyzed ROP reactions for [l]ferrocenophanes. A logical first step in the polymerization is insertion of the transition metal into the strained Cp-carbon-bridging element bond in the ferrocenophane. Polymers 93 formed in the presence of hydrosilanes are believed to result from competitive oxidative addition between the Si-H bond of the hydrosilane and the strained Gp-Si bond of the ferrocenophane at the catalytic center followed by reductive elimination. Detailed work has indicated that colloidal metal is the likely catalyst in the ROP reactions. 

This concept, leading to this new type of catalyst, is an extension to colloidal metals of the well-known heterogeneous enantioselective catalyst. Platinum sols stabilized by dihydrocinchonidine (Fig. 7) can be synthesized in different particle sizes by reduction of platinum tetrachloride with formie acid in the presence of different amounts of alkaloid. The resulting nanoparticles are enantioselective in the hydrogenation of ethyl 2-oxopropionate (ethyl pyruvate) optical yields are 75-80%... 

Copper colloids protected by PVPD with a 3240 degree of polymerization, most effectively catalyze the selective (100%) hydration of acrylonitrile (AN) to acrylamide in water at 80 °C at a molar Cu/AN ratio of 0.017. The acrylamide yield reaches 25.4 mole % within 2h [46]. The reaction is first order with respect to the acrylonitrile concentration down to 47% conversion. The catalytic activity of all other protected colloidal dispersions is also much higher (2.5-8.6 mole % in 2h) than the activity of the copper precipitate which forms by the reduction of copper sulphate by NaBH4 in the absence of a copolymer (0.3 mole % in 2 h). Hirai and Toshima [46] have reviewed the preparation and characterization of polymer-protected colloidal metal catalysts together with their characteristic properties and some of their applications. 

All of the above processes, in principle, afford the benefit of controlling the particle sizes of bimetallic systems that are eventually formed, whereas it may be more difficult to synthesize bimetallic colloids via traditional methods. Depending on the thermodynamic properties of the two metals, conventional catalyst preparation techniques (i.e., co-impregnation, co-precipitation) may result in the formation of a homogeneous alloy, segregation into pure monometallic phases, or a combination thereof. On the other hand, templating by dendrimers at least ensures that both metals are in close proximity with each other during complexation and after reduction. 

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

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