Computational Investigation of CO Adsorption and Oxidation on Iron-Modified Cerium Oxide

The mechanisms for the oxidation of CO catalyzed by Fe-modified CeO2 surfaces have been investigated using periodic density functional theory calculations corrected for the on-site Coulomb interaction by a Hubbard term (DFT + U). The following findings were made: (i) Fe is stable both as an adsorbed atom, Fe delta+ (delta < 2), on the surface and as a dopant, Fe3+, in the surface region. (ii) The Fe dopant facilitates oxygen vacancy formation, whereas Fe adatoms might suppress oxygen vacancy formation. (iii) In addition to physisorbed CO as on the clean surface, physisorbed CO2 and chemisorbed CO (carbonate, CO32-) species are observed on the Fe-doped CeO2(111) surface. Vibrational frequency calculations were carried to characterize these species, (iv) CeO2(111) containing positively charged Fe ions, either as supported isolated Fe delta+ adatoms [or small Fe-x(delta+) (x = 2-5) clusters] or as substitutional Fe3+ ions, was found to catalyze the conversion of CO to CO2. Incorporating Fe3+ ions into the ceria lattice as substitutional point defects can instead sustain a full catalytic cycle for CO oxidation and catalyst regeneration. The Fe dopant promotes multiple oxidations of CO without any activation energy, leading to O vacancy formation and CO2 desorption. Molecular O-2 adsorbs at the O vacancy, forming O adspecies that then drive CO oxidation and recover the stoichiometric Fe-doped CeO2 surface. A Bader charge analysis was carried to characterize the oxidation state of Fe ions along the catalytic cycle.