O 2 isotopic exchange in the presence of O − anion radicals on the FeZSM-5 surface

Abstract

Oxygen anion radicals O − play an important role in various physico-chemical processes, including those taking place on solid surfaces, where their behavior is especially difficult to follow. This work presents a detailed study on the behavior of O − radicals adsorbed on FeZSM-5 surface in the reaction of O 2 isotopic exchange carried out over a wide range of temperatures (198–513 K). High concentration of O − in this unique system (called alpha-oxygen) made it possible to identify two exchange mechanisms, R 0 and R 1 , operating in the presence of O α . The R 1 mechanism ( E = 15 kJ / mol ) dominates in the high-temperature region and can be represented by the following equation: 18O 2 + 16O − α ⇄ [O − 3] α ⇄ 16O 18O + 18O − α . This mechanism is convincingly supported by several arguments: the first order reaction with respect to O 2; the linear dependence of reaction rate on O α concentration; the ESR observation of O − 3 species (in other systems); and, according to isotopic exchange theory, by a twofold excess of the rate of homoexchange over the rate of heteroexchange. The R 0 exchange ( E = 0.8 kJ / mol ) is a new reaction pathway that appears at cryogenic temperatures and leads to isotope redistribution only between O 2 molecules, not affecting the isotopic composition of O α . This exchange mechanism is not quite clear. The necessity of the presence of O α without its direct involvement in the exchange process seems to be quite surprising. The reaction may proceed via a hypothetical complex [O − 5] α , which allows cleavage and redistribution of chemical bonds between two O 2 molecules adsorbed on the α-oxygen. The cleavage and redistribution of such strong bonds at cryogenic temperatures with nearly zero activation energy are an intriguing phenomenon worthy of further study.