Effect of the Mechanism of H2S on Elemental Mercury Removal Using the MnO2 Sorbent during Coal Gasification

A combination of experiments and density functional theory (DFT) calculations was employed to investigate the detailed reaction mechanism of elemental mercury (Hg-0) with H2S over a MnO2 surface. The MnO2 sorbent was prepared by a low-temperature sol gel autocombustion method and used to capture Hg-0 from simulated syngas. The experimental results show that MnO2 possesses superior Hg-0 removal capacity; over 85% Hg-0 removal efficiency is achieved in the temperature range of 80-200 degrees C. An appropriate concentration of H2S promotes Hg-0 removal by forming active sulfur species on the MnO2 surface. The computational results indicate that Hg-0 and HgS are chemically adsorbed on the MnO2 (110) surface, with the adsorption energies of -69.50 and -286.33 kJ/mol, respectively. H2S undergoes dissociative chemisorption on the MnO2 (110) surface and forms active sulfur species for Hg-0 transformation. Both Eley-Rideal (ER) and Langmuir-Hinshlwood (L-H) mechanisms are responsible for heterogeneous Hg-0 reaction with H2S over MnO2. The ER mechanism (7.16 kJ/mol) in which gaseous Hg-0 reacts with active surface sulfur species is kinetically more favorable than the L-H mechanism (42.00 kJ/mol) as a result of its much lower energy barrier. After the Hg-0 heterogeneous reaction, the most stable mercury compound of HgS formed on the MnO2 surface, which is verified by the temperature-programmed desorption and X-ray photoelectron spectroscopy experimental results.