O2 isotopic exchange in the presence of O− anion radicals on the FeZSM-5 surface

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 O2 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, R0R0 and R1R1, operating in the presence of Oα. The R1R1 mechanism (E=15 kJ/molE=15 kJ/mol) dominates in the high-temperature region and can be represented by the following equation:18O2 + 16O−α ⇄ [O−3]α ⇄ 16O18O + 18O−α. This mechanism is convincingly supported by several arguments: the first order reaction with respect to O2; the linear dependence of reaction rate on Oα 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 R0R0 exchange (E=0.8 kJ/molE=0.8 kJ/mol) is a new reaction pathway that appears at cryogenic temperatures and leads to isotope redistribution only between O2 molecules, not affecting the isotopic composition of Oα. This exchange mechanism is not quite clear. The necessity of the presence of Oα 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 O2 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.

High reactivity of O−/FeZSM-5 species (α-oxygen) in the oxidation of methane, benzene and other substrates at room temperature is well known. This work reveals high reactivity of α-oxygen in isotopic exchange with O2 at 198–448 K.Figure optionsDownload high-quality image (28 K)Download as PowerPoint slide