Mercury heterogeneous catalysis

Gas Phase. The decomposition of gaseous ozone is sensitive not only to homogeneous catalysis by light, trace organic matter, nitrogen oxides, mercury vapor, and peroxides, but also to heterogeneous catalysis by metals and metal oxides. 

Thus the peculiar titration results were due to the heterogeneous catalysis, not of the original reaction, but of a new reaction that had arisen from the analytical procedure. Precipitates formed by the reagents themselves can also prove catalytically active. For instance, the catalysis by rare earth metal salts of the hydrolysis of acid phosphonate esters was actually due to the development of metal hydroxide gels [159], A quite different example concerns reactions of mercury salts where the disproportionation... 

Barbosa et al. [41] studied the heterogeneous catalysis of this reaction in the classical solvent of 80 vol.% (55.2 mol.%) EtOH + H20 at 25°C, using a pH-stat technique to measure the rate. Silver, silver bromide, silver sulphide, mercury(I) bromide, and mercury(II) sulphide were all found to be good catalysts (cf. Fig. 12). With AgBr the catalytic rate was not affected by... 

The first vinyl ester prepared was vinyl acetate, in 1912, by homogeneous mercury sulfate catalysis [29]. For the vinyl esters of lower carboxylic acids, the reaction was soon carried out in the gas phase over, for example, zinc salts of the corresponding carboxylic acids (such as zinc acetate) heterogenized on active charcoal [30]. By 1965, vinyl acetate was prepared almost exclusively by this method. Since then, this synthesis has been largely superseded by the ace-toxylation of ethylene, the petrochemical preparation of which is highly economic (Section 2.1.4.1). 

With many electrode reactions, the adsorption of reactants, products, and/or intermediates controls the pathways as well as the reaction rates. Electrochemical reactions are part of the general field of heterogeneous catalysis (31). By controlling the chemical and structural features of the electrode surface (32) as well as electrolyte composition and potential, it is possible to achieve selectivity and specificity for electrochemical reactions. For example, the rate of generation of hydrogen on platinum is 9 to 10 orders of magnitude faster than on lead or mercury at potentials near the reversible thermodynamic value. 

Silver(i) is another catalyst for aquation. Unfortunately, a kinetic study of its catalytic effect on aquation of [CoI(NH8)5l + is complicated by heterogeneous catalysis by the silver iodide produced. Indeed this topic of heterogeneous catalysis of aquation of cobalt(iii) complexes has recently been reviewed and extended by studies of the kinetic effects of a range of diverse solids, e.g. mercury(ii) sulphide, silver bromide, and platinum metal. ... 

A few years ago the kinetics of heterogeneous catalysis of the aquation of the [Co(NHa)5Br] + cation were investigated - some of the catalysts employed [e.g. mercury(n) and silver(i) compounds] were closely related to the metal-ion catalysts discussed earlier in this section. A recent reinvestigation of some of these systems shows that part of the catalytic effect of these solids can be ascribed to redox processes operating in parallel to simple catalysed aquations. ... 

In heterogeneous electro catalysis, the catalyst is immobilized on the electrode surface, or the electrode itself plays the role of a catalyst. Catalytic effects of various electrode materials on the hydrogen evolution reaction are typical examples of heterogeneous electro -catalysis [iii]. Further examples are electrode mechanisms involving hydrogen evolution at a mercury electrode catalyzed by adsorbed organic bases, microparti-... 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

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