New reaction of anion radicals O− with water on the surface of FeZSM-5

Complexes of bivalent iron stabilized in the FeZSM-5 zeolite matrix (α-sites) are known to be able to decompose nitrous oxide stoichiometrically to form oxygen anion radicals, O−, bound to iron (α-oxygen),(FeII)α + N2O → (FeIIIO−)α + N2. Similar to O− radicals on V and Mo oxides, α-oxygen is highly reactive in respect to CO, H2, methane and other hydrocarbons. It participates in catalytic oxidation of benzene to phenol by nitrous oxide, providing selectivity close to 100%. In this work, adsorption measurements, IR spectroscopy, TPD, and isotope methods were used to describe an earlier unknown reaction of O− species with water, which proceeds at 5–200 °C via the hydrogen abstraction mechanism according to the equation2(FeIIIO−)α + H2O → 2(FeIIIOH−)α + (1/2)O2. The reaction leads to the hydroxyl groups formed on α-sites, and equivalent amount of O2 evolves into the gas phase from water. Desorption of the hydroxyl groups occurs at above 400 °C via their recombination into water and dioxygen, resulting in the reactivation of α-sites,2(FeIIIOH−)α → 2(FeII)α + H2O + (1/2)O2. High sensitivity of α-oxygen to water admixtures may be an important factor explaining some contradictory results reported in the literature on the behavior of α-oxygen. Because α-oxygen is a typical representative of anion radicals O−, this reaction may be relevant not only to the conventional oxidation catalysis over zeolites and metal oxides, but also to photocatalytic processes, where formation of O− is well documented.