Oxidative addition heterogeneous hydrogenation

The mechanism of inhibition by the salts of the long chain fatty acids has been examined . It was concluded that, in the case of the lead salts, metallic lead was first deposited at certain points and that at these points oxygen reduction proceeded more easily, consequently the current density was kept sufficiently high to maintain ferric film formation in addition, any hydrogen peroxide present may assist in keeping the iron ions in the oxide film in the ferric condition, consequently the air-formed film is thickened until it becomes impervious to iron ions. The zinc, calcium and sodium salts are not as efficient inhibitors as the lead salts and recent work has indicated that inhibition is due to the formation of ferric azelate, which repairs weak spots in the air-formed film. This conclusion has been confirmed by the use of C labelled azelaic acid, which was found to be distributed over the surface of the mild steel in a very heterogeneous manner. ... 

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. 

In heterogeneous catalysis the catalyst is present as a phase distinct from the reaction mixture. The most important case is the catalytic action of certain solid surfaces on gas-phase and solution-phase reactions. A critical step in the production of sulfuric acid relies on a solid oxide of vanadium (V2O5) as catalyst. Many other solid catalysts are used in industrial processes. One of the best studied is the addition of hydrogen to ethylene to form ethane ... 

There are numerous studies concerning solvent effects on selectivity in heterogeneous hydrogenations (1). Similar effects are observed in oxidation or acid catalysed reactions. In addition, molecular sieves can be viewed as solvents for the species that they contain (2) and therefore, even in vapor phase reactions, modifications of activity or selectivity can be attributed to solvent effects. 

Irrespective of the phase system (homogeneous, aqueous biphasic, heterogeneous single-site), the hydrogenation mechanisms of thiophene (T) or BT catalyzed by metal complexes comprise the usual steps of H2 oxidative addition, f-C,C coordination of the substrate, hydride transfer to form dihydrobenzothienyl, and elimination of DHBT by hydride/dihydrobenzothienyl reductive coupling (Scheme 2) [8],... 

The catalytic reduction of a particular intermediate involves the addition of hydrogen. Just as in oxidation catalysis, there is the issue of whether one or two hydrogen atoms from H2 are incorporated into the substrate through identical intermediate hydrogen atoms. In heterogeneous catalysis, this question translates into whether dissociation occurs ho-molytically or heterolytically. On a transition-metal surface H2 dissociation generates two equivalent hydrogen atoms such as we have seen in Chapter 3. As discussed in Chapter 4, however, Hg can dissociate heterolytically H2 H + H+. 

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