Mechanism of the Formation of O2− Radical Anions on CeO2 and (0.5–10)%CeO2/ZrO2 during the Adsorption of an NO-O2 Mixture

It is established by ESR that the adsorption of an NO + O2 mixture at 20°C on oxidized CeO2 (O2, T = 400–700°C) produces radical anions O2− located both on isolated Ce4+ cations (O2−(1)) and in associated anionic vacancies (O2−(2)). These species differ in thermal stability. For example, O2−(2) decomposes at 20°C, while O2−(1) decomposes at 50°C. Only O2− (1) species are observed at −196°C in ZrO2-supported CeO2. In the case of NO + O2 adsorption at 20°C, O2− is stabilized on Zr4+ cations and decomposes at 270°C. Increasing the cerium oxide content of the ZrO2 surface from 0.5 to 10% only partially inhibits the formation of O2−-Zr4+. The Zr4+ cation is shown to possess a higher Lewis acidity than the Ce4+ cation, and the ionic bond in O2−-Zr4+ complexes is stronger than that in O2−-Ce4+ complexes. ESR, temperature-programmed desorption, and IR spectroscopic data for various adsorption complexes of NO on CeO2 suggest that, in the key step of O2− formation, free electrons appear on the surface owing to the conversion of adsorbed NO molecules into nitrito chelates on coordinately unsaturated ion pairs Ce4+-O2−.