Oxygen Species involved in Partial Oxidation

Copper.—Oxygen Species involved in Partial Oxidation. The reaction of propene with copper oxide in the absence of gas-phase oxygen has recently been investigated by Anshits et al. Starting with copper(ii) oxide, they obtained a zero selectivity to acrolein and a low rate of reaction at 423 K using a propene pressure of less than 1 Torr. As the catalyst was reduced, the activity peaked at a catalyst composition corresponding to CuOo.9. A peak selectivity to acrolein of 75 % was achieved on a supported copper oxide when the composition was CuOo.5. Further reduction resulted in a sharp loss of both activity and selectivity. 

The same group of workers have also studied propene oxidation in the presence of oxygen, again at pressures below 1 Torr and at relatively low temperature (473 The partial and complete oxidation reactions both showed a first- 

Contrary evidence, that adsorbed oxygen is involved in the partial oxidation of propene, has been obtained using labelling. When a pulse containing propene and 0-enriched oxygen was passed over a Cu2 0 catalyst at 623 K the ratio of in the acrolein produced was identical to the 0  

Akimoto, M. Akiyama, and E. Echigoya, Bull. Chem. Soc. Japan, 1976, 49, 3367. 

In summary, the available experimental evidence suggests that an adsorbed form of molecular oxygen is involved in partial oxidation while lattice oxygen is required for carbon dioxide production. This proposed mechanism is directly opposed to that generally accepted for propene oxidation over mixed oxide catalysts such as bismuth molybdate. In this case, lattice oxygen is responsible for acrolein formation while adsorbed oxygen results in complete combustion. This means that the fully oxidized phase is the selective catalyst while the reaction is first order with respect to alkene. 

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H-abstraction on surface 0 species has been generally proposed, while it is also accepted that the oxygen feed does not participate directly in the reaetion. In any case, the nature of oxygen species involved in partial oxidation reaetions will depend on the nature of active sites and the reaction conditions. Thus, peroxo and super-0x0 species could also be present on the catalyst surface generating different active sites. The 0 species formation would be related to struetural defeets sueh as excess oxygen anions or cationic vacancies, revealed by the p-type semi-conduetion properties of the catalysts, or to charge transfers — 0 ] thermally... 

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