Fe-doped CeO2 solid solutions: Substituting-site doping versus interstitial-site doping, bulk doping versus surface doping

Doping CeO2 cubic fluorite with transitional metal ions can effectively improve its redox behavior, oxygen storage capacity and catalytic performance, but the relevant fundamental understanding of the promotion effect is still insufficient due to the difficulty on determining the distribution of dopant. We herein demonstrate an effective approach to determine this dopant distribution by combining X-ray absorption spectroscopy and selective chemisorption. Cubic CexFe1-xO2 fluorite solid solutions (x ≥ 0.70) were prepared by co-precipitation method. With the increasing of Fe molar ratio in CexFe1-xO2, Fe3+ initially substitutes Ce4+ and/or occupy intersitial sites with x ≥ 0.80, and then transfers to form sub-Fe2O3 structure in fluorite lattice as more Fe3+ are present; meanwhile, the Fe3+ doping initially occurs only in the bulk with x ≥ 0.96 and then extends to the surface with 0.87 ≤ x < 0.96. Low calcinations temperature facilitates the doping of Fe3+ in the bulk of cubic CexFe1-xO2 solid solutions. These results reveal the structures of CexFe1-xO2 fluorite solid solutions at the molecular level that are of great importance for the fundamental understanding of their properties.