Superoxide and Peroxide Species on CeO₂(111), and Their Oxidation Roles

The electronic properties and oxidation roles of superoxide and peroxide species on the small and big CeO 2 (111) surfaces with an oxygen vacancy are systematically investigated utilizing density functional theory. When the CeO 2 surface is partially reduced, a surface oxygen vacancy forms, and two Ce 4+ ions of substrate are reduced to Ce 3+ ones. If O 2 adsorbs at the top site of Ce 3+ ion, which is close to the surface oxygen vacancy, it slips into the vacancy. Then, two 4f electrons of Ce 3+ ions feedback to the π2p* orbital of O 2, and a diamagnetic peroxide species forms. When O 2 adsorbs at the top site of Ce 3+ ion apart from the surface oxygen vacancy, superoxide species with three different orientations are obtained. Meanwhile, only one Ce 3+ ion is oxidized to Ce 4+ since one 4f electron transfers to the π2p* orbital of O 2 . The migration barrier from O 2 – (η 2 -1) to O 2 2– is 0.35 eV, indicating that superoxide might easily transform into peroxide with elevating temperature. CO is directly oxidized to CO 2 by the superoxide without energy barrier, while a carbonate forms when CO reacts with peroxide. A high desorption barrier of the carbonate to form a gas CO 2 molecule indicates that peroxide species might play the dominant role at relatively high temperature.