Catalytic Role of O•– Radicals in the Low-Temperature Isotopic Exchange in Dioxygen

Abstract

In spite of a long investigation history, the low-temperature isotopic exchange in dioxygen taking place without involvement of the catalyst oxygen (R0 exchange) is still an exotic and poorly understood phenomenon in heterogeneous catalysis. Although very strong bonds are to be cleft in O2 molecules (118 kcal/mol), over some metal oxides R0 can be observed even at the temperature of liquid nitrogen. In this work, we studied the R0 exchange over a FeZSM-5 zeolite at 233 K and discovered for the first time a linear dependence of the R0 rate on the concentration of O•– radicals (α-oxygen), which identifies a catalytic role of these species. Upon transition to cryogenic temperatures, O•– species lose the ability to exchange themselves with dioxygen, but start functioning as a principal part of unique active sites capable of catalyzing a very smooth redistribution of the oxygen–oxygen bonds in adsorbed O2 molecules. Running by a highly concerted mechanism, this remarkable process leads to the R0 exchange with ...