Silver ions as catalyst

Homogeneous oxidants Peroxydisulfate ion, S20g (often called persulfate), is a strong oxidant in acid solution in the presence of silver ion as catalyst. The reaction... 

Oxidative decarboxylation of acetic acid at 75 °C by (NH4)2S20g and silver ions as catalyst in 10% H2SO4 containing 3-methyl-IbP 41 led to the formation of a mixture of variously C-methylated IbPs. Gradually increasing the excess of the generated methyl radicals with respect to the substrate first gave a mixture of di- and trimethyl-IPs 295 and 296 (in 71% overall yield), and then 2,3,5,7-tetramethyl-IbP 297 in 6% yield. 

Mn(II) can be oxidized quantitatively to permanganate in perchloric acid by the use of silver ion as a catalyst. In the presence of phosphate, Ce(III) is oxidized to Ce(lV) phosphate, which precipitates from a solution containing sulfuric and phosphoric acids. The precipitate can be dissolved in sulfuric acid. Other oxidations that can be performed quantitatively are V(IV) to V(V) in acid solution, hypophosphite and phosphite to phosphate, selenite to selenate, tellurite to tellurate, nitrite to nitrate, and in alkaline solution, iodide to periodate. 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

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. 

As is the case for [2 + 2] cycloaddition reactions (15-61), certain forbidden electrocyclic reactions can be made to take place by the use of metallic catalysts." An example is the silver ion-catalyzed conversion of tricyclo[4.2.0.0. ]octa-3,7-diene to cyclooctatetraene " ... 

Colloids of more electronegative metals such as cadmium and thallium also act as catalysts for the reduction of water. In the colloidal solution of such a metal, an appreciable concentration of metal ions is present. The transferred electrons are first used to reduce the metal ions, thus bringing the Fermi potential of the colloidal particles to more negative values. After all the metal ions have been reduced, excess electrons are stored as in the case of silver. 

Fig. 1. Reduction of silver ions by hydroquinone 1, no catalyst added 2, qui-none added 3, silver sol added 4, gold sol added 5, palladium sol added 6, silver sulfide sol added. Gelatin (1 %) was present as stabilizer.

The same considerations can be applied to the mechanism of the hydroxylamine-silver ion reaction as have been given already for the hydroquinone-silver ion reaction. Bagdasar yan s equation adequately expresses the dependence of rate on the silver ion concentration and on the surface of the catalyst, but not the dependence on the hydroxylamine concentration. The latter dependence, on the other hand, would be in agreement with the assumption that hydroxylamine is adsorbed by the silver prior to reaction. 

The kinetics of the reduction of silver ions by p-phenylenediamine differ in important respects from those of the reduction by hydroquinone and hydroxylamine. Once more, the silver catalysis is marked and the reaction rate varies directly as the area of the catalyst surface, but the rate is directly proportional to the silver ion concentration (James, 7). 

Conditions existing at the triple interface, Ag/AgX/Solution, will influence the rate at which the catalyzed reaction occurs. These conditions will determine the activity of the silver ions, the concentration of adsorbed developer, and the state of the catalyst. The halide ion will be an important factor in determining the activity of the silver ions, and Sheppard has suggested that the hydration and diffusion away of the halide ion is the dominant factor in determining the specific rate of reaction at the interface (Sheppard, 15). The known order of reactivity of the halides, AgCl > AgBr > Agl, follows as a natural consequence from this point of view, whereas it would not be predicted on the basis of the electrode mechanisms. 

A trimethylsilylethynyl moiety can serve as a vinyl cation equivalent, as shown by an intramolecular reaction with a dienamine [125], The silver ion catalyst acts as an accentuating agent. 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

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