First-principles calculations were performed to investigate the mechanism of Hg-0 adsorption and oxidation on CeO2(1 11). Surface oxygen activated by the reduction of Ce4+ to Ce3+ was vital to Hg-0 adsorption and oxidation processes. Hg-0 was fully oxidized by the surface lattice oxygen on CeO2(111), without using any other oxidizing agents. HCl could dissociate and react with the Hg adatom on CeO2(111)to form adsorbed Hg-Cl or Cl-Hg-Cl groups, which promoted the desorption of oxidized Hg and prevented CeO2 catalyst deactivation. In contrast, O-H and H-O-H groups formed during HCl adsorption consumed the active surface oxygen and prohibited Hg oxidation. The consumed surface oxygen was replenished by adding O-2 into the flue gas. We proposed that oxidized Hg desorption and maintenance of sufficient active surface oxygen were the rate-determining steps of Hg-0 removal on CeO2-based catalysts. We believe that our thorough understanding and new insights into the mechanism of the Hg-0 removal process will help provide guidelines for developing novel CeO2-based catalysts and enhance the Hg removal efficiency.