Colloidal Metals Catalysts, forms

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... 

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. 

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. 

Contrary to previous reports suggesting colloidal metal as the active species in Pt-catalyzed hydrosilylations, the catalyst was found to be a monomeric platinum compound with silicon and carbon in the first coordination sphere.615 The platinum end product at excess olefin concentration contains only platinum-carbon bonds, whereas at high hydrosilane concentration, it is multinuclear and also contains platinum-silicon bonds. An explanation of the oxygen effect in hydrosilylation was also given to show that oxygen serves to disrupt multinuclear platinum species that are formed when poorly stabilizing olefins are employed. 

Solvated metal atoms can be dispersed in excess organic solvent at low temperature and used as a source of metal particles for the preparation of both unsupported metal powders and supported metal catalysts158,161. Alternatively, metal vapor is condensed into a cold solution of a stabilizing polymer to form crystallites of the order 2-5 nm in diameters159. Equation 17 illustrates the unique activity of a colloidal Pd catalyst in the partial hydrogenation of acenaphthene. 

The aerobic oxidation of alcohols is catalysed by both low- and high-valent forms of the metal. In the former case the reaction involves (Fig. 5) the formation of a hydridometal species (or its equivalent), while the latter involves an oxometal intermediate (Fig. 6) which is regenerated by reaction of the reduced form of the catalyst with dioxygen instead of a peroxide. It is difficult to distinguish between the two and one should bear in mind, therefore, that aerobic oxidations with high-valent oxometal catalysts could involve the formation of low-valent species, even the (colloidal) metal, as the actual catalyst. 

It was found, that also Ru and Os colloids can act as catalysts for the photoreduction of carbon dioxide to methane [94, 95]. [Ru(bpy)3]2+ plays a role of a photosensitizer, triethanolamine (TEOA) works as an electron donor, while bipyridinium electron relays (R2+) mediate the electron transfer process. The production of hydrogen, methane, and small amounts of ethylene may be observed in such a system (Figure 21.1). Excited [Ru(bpy)3]2+ is oxidized by bipyridinium salts, whereas formed [Ru(bpy)3]3+ is reduced back to [Ru(bpy)3]2+ by TEOA. The reduced bipyridinium salt R + reduces hydrogen and C02 in the presence of metal colloids. Recombination of surface-bound H atoms competes with a multi-electron C02 reduction. More selective reduction of C02 to CH4, ethylene, and ethane was obtained using ruthenium(II)-trisbipyrazine, [Ru(bpz)3]2+/TEOA/Ru colloid system. The elimination of hydrogen evolution is thought to be caused by a kinetic barrier towards H2 evolution in the presence of [Ru(bpz)3]2+ and noble metal catalysts [96]. 

Biologically important substrates can be hydrogenated in aqueous solution by using sulfonated triarylphosphine ligands that confer water solubility upon the catalyst. However, it was later reported that rhodium metal was formed from these complexes in the presence of hydrogen. It was claimed that the role of the ligands is merely to prevent aggregation of the rhodium colloid produced. 

The very soluble hexachloroplatinic acid, H2 [PtCl ] nH20, is the most useful precursor for synthetic and catalytic work. For the hydrosilation of unsaturated substrates, the catalyst of choice is chloroplatinic acid, because of its very high activity. There is currently some doubt as to whether the true catalyst is homogeneous or colloidal metal and therefore heterogeneous see Hydrosilation Catalysis). Nearly all halides andpseudohahdes (= X) form salts of the [PtXe] ion and of the [PtX4(NH3)2] type. 

Manners first proposed that the transition-metal-catalyzed ROP occurred via a homogenous mechanism.157 However, a heterogenous catalytic cycle has been reported.158 The proposed mechanism for the Pt(l,5-cod)2 (cod-cyclooctadiene) catalyzed reaction is shown in Scheme 2.24. The Pt(l,5-cod)2 forms a [2]platinasilaferrocenophane through oxidative addition to the zero-valent Pt complex via elimination of a 1,5-cod ligand. Platinum colloids are then formed by the elimination of the second 1,5-cod ligand these platinum colloids are proposed to be the active catalysts. The polymers are then formed by subsequent oxidative addition and reductive eliminations at the colloid surface. 

The extrusion method for catalyst forming led to interesting mechanical properties. The resistance towards crushing of extrudates was around 120 N/cm compared to a value below 5 N/cm for the catalyst reference sample. The material porosity decreased due to the introduction of the colloidal silica particles in the catalyst porosity. The problem is that the catalytic activity of the extrudates is less than half that of the reference material. The activity decrease could arise because of porosity filling in by the colloidal silica particles making the metal particles inaccessible for reaction. 

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. 

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