Catalysis silver surfaces

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

Thus, the available evidence indicates that little or no adsorption of hydroquinone by silver occurs. Rabinovich s data are unacceptable because of the large experimental errors involved. The possible amount of adsorption indicated by the data of Perry, Ballard, and Sheppard does not exceed the limits of error in their analytical determination of hydroquinone and could not under any circumstances cover more than a small fraction of the silver surface. The kinetics of the reaction between hydroquinone and silver ions do not indicate adsorption of the reducing agent, although the first-order dependence of rate on concentration is not incompatible with weak adsorption. It seems unlikely, accordingly, that adsorption of hydroquinone by silver plays a role of any consequence in the silver catalysis of the reaction between hydroquinone and silver ion. 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Later work by Stevenson [72] supported this hypothesis. The preparation of PET catalysed by antimony trioxide was studied in thin films on metal surfaces that were carefully selected to avoid catalysis by surface effects or by dissolved metal as mentioned earlier, a large number of metals and their oxides, salts or other derivatives catalyse the polyesterification reaction. On inactive surfaces like silver or rhodium the catalysed polycondensation rate increased with decrease in film thickness. In the absence of added catalyst there was no tendency for the rate to increase with decreasing film thickness. Stevenson proposed that in thin films the catalyst-deactivating component was more readily lost, thereby increasing the reaction rate. 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

The structure of metal clusters is of importance to organometallic chemistry, catalysis and surface science. Silver clusters form the latent photographic image135. 

Catalyzed gaseous systems. The reaction between CO and CO2 at elevated temperatures ( 860-920°C) represents a classic example of carbon isotope exchange enhanced by catalysis with surfaces such as quartz, gold and silver (Brander and Urey 1945, Hayakawa 1953). In addition to rate enhancement on catalytic metal or oxide surfaces, Brander and Urey (1945) also observed an increase in rate in the presence of H2 or H20(v). They tested a variety of mechanisms to explain their experiments which involved reaction of C-enriched CO2 with normal CO as a function of temperature, gas... 

These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from HjO and 0(a) was recently reported 145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. 

In the oxidation of hydroxylamine by silver salts and mercurous salts, the nature of the reaction product apparently depends upon the extent to which catalysis participates in the total reaction. This is illustrated by some results obtained with mercurous nitrate as oxidizing agent. The reaction is strongly catalyzed by colloidal silver, and is likewise catalyzed by mercury. The reaction of 0.005 M mercurous nitrate with 0.04 M hydroxylamine at pH 4.85 proceeds rapidly without induction period. The mercury formed collects at the bottom of the vessel in the form of globules when no protective colloid is present, so the surface available for catalysis is small. Under these conditions the yield is largely nitrous oxide. Addition of colloidal silver accelerates the reaction and increases the yield of nitrogen. Some data are given in Table III. 

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). 

However, organic pollutants are often accompanied by heavy metal ion contaminants that can be reduced by photogenerated electrons into their less toxic, nonsoluble metallic form. Ti02-assisted photoreductive catalysis was found to be useful in the removal of certain heavy metals including mercury, silver, platinum, palladium, rhodium, and gold via their reduction followed by deposition at the catalyst surface [20-22] or photoreduction of nitroaromatic compounds [23-26]. The use of photogenerated electrons for deposition of metal layers on... 

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