A study of the structure sensitivity of the exchange of oxygen with the surface of magnesium oxide

Abstract

The kinetics of exchange between 18 O 2 and the surface of magnesium oxide have been studied for four samples with surface areas of between 10 and 100 m 2 g −1. Less than 20% of the MgO oxygen monolayer undergoes exchange, and reaction is first order in exchangeable oxygen. Specific rate constants are very similar on all four samples, with all of the activation energies in the range 129±9 kJ mol −1, indicating that the exchange reaction is structure insensitive. After exhaustive exchange of the MgO surface with pure 18 O 2 , the gas phase was changed to 16 O 2 . Surprisingly, production of 18 O 16 O restarted, at a rate identical to that originally observed between 18 O 2 and the surface. This fascinating result indicates that, although the reaction kinetics show that heterolytic oxygen exchange is structure insensitive, there are nevertheless unique surface sites for the exchange reaction, which are not destroyed or randomised during the exchange process. Possible involvement of surface hydroxyl groups in the mechanism of exchange is discussed.