Examination of oxygen vacancy formation in Mn-Doped CeO_2 (111) using DFT+U and the hybrid functional HSE06

摘要

MnO_x-CeO_x mixed oxide systems exhibit interesting sulfur adsorption capacities and catalytic activity. We examined the electronic structure of Mn-doped fluorite CeO_2 bulk solid and surface using density functional theory (DFT) with the Hubbard U term or the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. We specifically evaluate the reducibility and formation energies of Mn-doped CeO_2 surfaces. The use of a U value on the d-states of Mn is examined, and a value of 4 eV is chosen based upon results from DFT+U calculations on bulk MnO_x,1 XANES characterization of oxidation states in calcined and reduced Mn-doped CeO_2, and comparison with HSE06 hybrid functional results. Electronic structure impacts of the U inclusion are discussed. The concentration and orientation of Mn atoms doped into the surface of CeO_2 have a great influence on the reducibility of the surface. Based upon formation energies, Mn will not favor doping into the surface of CeO_2 in a fully oxidized system (Mn~(4+)). Under reducing environments, Mn will dope into the surface with oxygen vacancies present (Mn~(3+) and Mn~(2+)). The first oxygen vacancy is not likely catalytically important in fluorite MnO _x-CeO_x systems as formation of the fully oxidized surface is not stable. A greater degree of reduction would occur during a catalyzed redox reaction.