Exchange and Adsorption of Oxygen Species

Khalif et al.15 measured microcalorimetrically the heat of chemisorption of 02 on V205 supported on MgO and A1203. On the first 02 and O- are formed with heats of adsorption of 83 kJ mol-1 and of 245 kJ mol-1 and the latter oxidizes CO at 25 °C. On A1203 the heat of adsorption exceeds 250 kJ mol-1 and 02 ions are not formed. 

Shelimov and Che68 used e.s.r. to study the kinetics of 02 exchange involving adsorbed O- ions on V205/Si02 and found a rate constant of 0.9 x 10-19 cm3 mol-1 s-1 while only about 70% of the adsorbed O species participate in the process, in contrast to reactions with H2 and CO which already proceed at temperatures as low as 77 K. 

Bielanski et al.18 studied the redox processes occurring upon interaction of 02 and propene with V2Os. Below 200°C with 02 and below 100°C with propene, the reactions are restricted to the surface layer. For the reoxidation of the reduced catalyst to be fast, the presence of some V3+ ions is essential as these are oxidized much more quickly than V4+ ions, the activation energies for the oxidations being 6.3 and 21 kJ mol-1 respectively. Aso et 

Yoshida et alP2 observed that oxygen within a layer of 20 A thickness of the surface of copper vanadate catalysts takes part both in reduction by CO and in CO oxidation. The Cu ions are also very mobile in this layer and surface enrichment of these ions occurs upon 02 treatment after reduction at 200°C. 

Andersson developed a semi-empirical model for the charge distribution around the (V=0) bonds in V205, V6013, and V02.73 The surfaces of the lower oxides were treated, upon the basis of ESCA results discussed on p. 107, as being in an oxidized state, which is proposed to be the case under the usual conditions in (amm) oxidation reactions. The main result is that 02-in the form of (V=0) groups is responsible for the catalytic oxidation of hydrocarbons. 

---