Heterogeneous reaction mechanism of elemental mercury oxidation by oxygen species over MnO2 catalyst

MnO2-based catalysts have attracted great attention in the field of elemental mercury (Hg-0) catalytic oxidation because of their superior catalytic performance and wide temperature window. Quantum chemistry calculations based on density functional theory (DFT) combined with periodic slab models were carried out to investigate the heterogeneous mechanism of Hg-0 oxidation by oxygen species (gas-phase O-2, chemisorbed oxygen, and lattice oxygen) on MnO2 surface. The results indicate that Hg-0 and HgO are chemically adsorbed on MnO2 surface with the adsorption energies of -69.50 and -226.48 kJ/mol, respectively. The adsorption of O-2 on MnO2 surface belongs to chemisorption, O-2 can decompose on MnO2 surface with an energy barrier of 97.46 kJ/mol to produce two atomic adsorbed oxygen. The perpendicular adsorbed O-2 and dissociative adsorbed O-2 are more favorable for Hg-0 catalytic oxidation than lattice oxygen, and perpendicular adsorbed O-2 is the most active oxygen for Hg-0 oxidation. The reaction pathway of He oxidation by perpendicular adsorbed O-2 includes three reaction steps: Hg-0 -> Hg(ads) -> HgO(ads) -> HgO. The third step (HgO(ads) -> HgO) is endothermic by 168.17 kJ/mol with an energy barrier of 179.48 kJ/mol, and it is the rate-limiting step of the whole Hg-0 oxidation reaction. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.