A comparative DFT+U study of CO oxidation on Pd- and Zr-doped ceria

Abstract

Metal-doped ceria catalysts have been applied in many important catalytic processes. In this work, we performed density functional theory calculations corrected by on-site Coulomb interactions to study the Pd- and Zr-doped CeO2(111) surfaces with the dopant at different locations. The formation of oxygen vacancies and CO oxidation were systematically calculated on the various doped surfaces. We find that both Pd and Zr doping can activate the surface lattice O and reduce the energy barriers of CO oxidation. However, the promotion effect of the Zr dopant is limited to its existence in the first surface layer, while for the Pd dopant, the surface activity can be greatly enhanced even it occurs far below the surface. Besides, CO2 can be generated directly on the Pd-doped surfaces through reaction between CO and surface O, while the surface intermediate CO2δ– may readily form and restrict the releasing of CO2 by further oxidation to carbonates on the Zr-doped surfaces. Electronic analyses show that the doped Pd exists as Pd4+ and it has stronger electron affinity than other surface species during CO oxidation, contributing to the easy Pd4+ to Pd2+ transformation accompanied by direct CO2 formation at Pd-doped ceria.