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

CuMn2O4 spinet has been experimentally demonstrated to be a kind of promising sorbent for Hg-0 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 Hg-0 removal by a CuMn2O4 sorbent, including active sites and the reaction mechanism. DFT calculation results show that Hg-0 and HgO adsorption on the CuMn2O4 surface is dominated by the chemisorption mechanism. The stronger interaction between Hg-0 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 O-2-activation ability due to the lower energy barrier. O-2 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 O-2 dissociation reacts with adsorbed Hg-0 to form HgO species. O-2* species can also directly react with adsorbed Hg-0. The most favorable pathway for gaseous HgO formation is described by a three-step process, namely He adsorption, Hg-0 oxidation, and HgO desorption. In the comprehensive mercury adsorption-oxidation-desorption process, HgO desorption is predicted to be rate-limiting.