Mercury removal during chemical looping combustion for coal is important in reducing elemental mercury emission. However, the oxidation mechanism of elemental mercury by oxygen carriers is still unclear. Mercury removal experiments were carried out in a fixed bed reactor using Co₃O₄ as the oxygen carrier, with online measurement of Hg⁰ concentration at the outlet of the reactor. Hg⁰ catalytic oxidation mechanism was also revealed through thermodynamic calculation. Experimental results show that both the homogeneous reaction between HCl and Hg⁰, and the heterogeneous reaction between HCl and Co₃O₄ contribute significantly to Hg⁰ removal. With the reduction of Co₃O₄, the removal efficiency on Hg⁰ drops dramatically. Calculation results show that three reaction routes exist for Hg⁰ oxidation: the reaction between HCl and Co₃O₄ produces Cl, Cl₂, and CoCl₃, which then reacts with Hg⁰ to form HgCl₂; Hg⁰ reacts with Cl and CoCl to form HgCl, which is then oxidized by Cl, Cl₂, and CoCl₃ to form HgCl₂; Hg⁰ reacts with lattice oxygen to form HgO, which then reacts with HCl to form HgCl₂. Overall, the contributions of different Cl-containing components for Hg⁰ oxidation follow the sequence of Cl₂ > Cl > CoCl₃. This attempt reveals the Hg⁰ oxidation in a furnace and sheds light on the development and application of Hg⁰ removal technology.