This study investigated the adsorption of elemental mercury (Hg) on perfect and reduced Fe2O3[104] surfaces as well as analyzed the synergistic effect of Hg on the catalytic decomposition of CO on the reduced surfaces using density functional theory calculations. This study aimed to clarify the correlations among Hg-0 adsorption, CO decomposition, and Fe2O3 reduction degree. Theoretical results indicated that Hg(0)underwent Fe top site adsorption to Fe bridge site adsorption as Fe2O3 was reduced into iron. This phenomenon increased E-ads with a local extreme of -2.063 eV for Hg-0-Fe2O1.364. The adsorption of Hg(0)decreased E-ads for CO adsorption on the reduced Fe2O3 surfaces but promoted charge transfer from the surface to the adsorbed CO molecule. This event activated the C O bond, favoring its decomposition. A kinetic model for catalytic CO decompositions with and without Hg further revealed the synergistic effect of Hg on carbon deposition and the relationship between the reaction rate and degree of chemical looping combustion.