Nature of Active Sites and an Oxygen-Assisted Reaction Mechanism for Mercury Capture by Spinel-Type CuMn2O4 Sorbents

CuMn2O4 spinel has been experimentally demonstrated to be a kind of promising sorbent for Hg0 capture from flue gas due to its excellent adsorption performance, regenerability, and recyclability. Theoretical studies based on the state-of-the-art density functional theory (DFT) were performed to gain an understanding of several important aspects of Hg0 removal by a CuMn2O4 sorbent, including active sites and the reaction mechanism. DFT calculation results show that Hg0 and HgO adsorption on the CuMn2O4 surface is dominated by the chemisorption mechanism. The stronger interaction between Hg0 and the CuMn2O4 surface is closely associated with orbital hybridization between the Hg atom and surface metal atoms (Cu and Mn). CuMn2O4 shows an excellent O2-activation ability due to the lower energy barrier. O2 dissociation reaction on the CuMn2O4 surface is activated by 6.31 kJ/mol and is exothermic by 101.37 kJ/mol. The chemisorbed oxygen atom produced from molecular O2 dissociation reacts with adsorbed Hg0 to form HgO species. O2* species can also directly react with adsorbed Hg0. The most favorable pathway for gaseous HgO formation is described by a three-step process, namely Hg0 adsorption, Hg0 oxidation, and HgO desorption. In the comprehensive mercury adsorption–oxidation–desorption process, HgO desorption is predicted to be rate-limiting.