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 CeO2(111) surfaces with an oxygen vacancy are systematically investigated utilizing density functional theory. When the CeO2 surface is partially reduced, a surface oxygen vacancy forms, and two Ce4+ ions of substrate are reduced to Ce3+ ones. If O-2 adsorbs at the top site of Ce3+ ion, which is close to the surface oxygen vacancy, it slips into the vacancy. Then, two 4f electrons of Ce3+ ions feedback to the pi 2p* orbital of O-2 and a diamagnetic peroxide species forms. When O-2 adsorbs at the top site of Ce3+ ion apart from the surface oxygen vacancy, superoxide species with three different orientations are obtained. Meanwhile, only one Ce3+ ion is oxidized to Ce4+ since one 4f electron transfers to the pi 2p* orbital of O-2. The migration barrier from O-2(-)(eta(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 CO2 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 CO2 molecule indicates that peroxide species might play the dominant role at relatively high temperature.