Investigation of Bi-doping effects on the catalytic performance of Pt/3DOM-Ce_1−xBi_xO_2−δ in the aerobic oxidation of 5-hydroxymethyl-2-furfural allows us to reveal the promoted catalytically active sites: the asymmetrical oxygen vacancies coordinated with one Bi and up to three Ce cations, such as Bi-□(-Ce)₃, where □ represents an oxygen vacancy, which can easily gain oxygen atoms in favor of the CeO₂ structure, and, when filled with oxygen, easily release oxygen anions in favor of six-coordination for Bi³⁺. The loss of electrons in the reduction of oxygen atoms at these sites would be replenished by electron transfer from Pt nanoparticles eventually promoting the oxidation potential of the Pt nanoparticles. The present work points out that the promoted catalytic properties in Bi-doped CeO₂ are mainly due to the asymmetric structures of the oxygen vacancies, rather than simply to the number of oxygen vacancies. The newly proposed model of asymmetrical active sites and electron transfer mechanism may shed light on the physicochemical properties of other solid solution substrate-supported  metal nanoparticle catalysts.