1 -02 = 0]

In this case the reaction is hindered by the adsorption of products.

As shown in Section II of this chapter, all reactions of catalytic oxidation can be categorized as electrophilic or nucleophilic ones. It is the nucleophilic addition of oxide ions 0 2-which results in the formation of oxygenated product molecules without destruction of the carbon skeleton in reactions which are called selective oxidation. They include such important industrial processes as oxidation of olefins to oxygenated allylic compounds (e. g., oxidation of propene to acrolein or acrylic acid, or oxidation of butene to maleic anhydride) and oxidation of functional groups in aromatic molecules (e. g., oxidation of toluene to benzaldehyde). However, thermodynamic data concerning the formation of different oxygen species, discussed in Chapter 2, indicate that formation of an oxide ion 0 2-requires the expenditure of very large amount of energy, this ion being stabilized only by the Madelung potential of the crystal lattice. It may thus be concluded that it is the lattice oxide ions, located at the surface of the oxide, which must be involved in nucleophilic oxidation.