Mechanism of the Reaction on Oxides

It is convenient to subdivide the low temperature and medium range temperature oxidation reactions according to the character of the oxygen active species  

The type (2) reactions are frequently of a very high selectivity, which is widely exploited by industry (about 20% of all chemicals are produced by oxidation reactions). The type (2) reactions proceed by the so called Mars and Van Krevelen mechanism [117], which is schematically shown for an oxidation reaction in Fig. 4.55. 

Oxides are more complicated systems than metals. However, with oxides, too, some information on the molecular level mechanism is available. For example, it has been established that variously coordinated active metal ions posses a different activity [121], For selective oxidation and reduction, the octahedral oxygen is frequently more reactive than the tetrahedral one. The oxidation reactions are also known to be crystal face sensitive or even specific [122-124]. 

Oxidation reactions (and the reversed reactions of the selective removal of oxygen, too) are practically very important. In the past, attention was mainly concentrated on oxidation of olefins, but nowadays research is focused on saturated hydrocarbon molecules. Some important reactions are listed in Table 4.3. 

Silver is an important metallic catalyst for the selective oxidation of ethylene. The silver catalyst is used to selectively convert ethylene to ethylene epoxide, an important intermediate for antifreeze. Whereas the epoxidation of ethylene proceeds with high selectivity on oxidic silver phases, metallic silver surfaces give only total oxidation of ethylene. Electron-deficient O is created on oxidized silver surfaces and this readily inserts into the electron-rich ethylene bond. 

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