Ag doping Fe-Ti spinel sorbent for Hg0 removal from syngas and the mechanism investigation

Abstract

A series of Ag doping Fe-Ti spinel sorbents (Ag/Fe-Ti spinel) were synthesized using co-precipitation and impregnation methods and employed to remove elemental mercury (Hg0) from syngas at high temperatures (200–350 °C). The role of H2S on Hg0 elimination over Ag/Fe-Ti spinel and the Hg0 removal mechanism were systematically investigated by experimental and theoretical methods. N2 adsorption–desorption, SEM-EDS, XRD, H2-TPR and XPS were used to characterize the physicochemical properties of samples. The synthesized Ag/Fe-Ti spinel was tested on a fixed-bed reactor for Hg0 removal from the simulated syngas and showed an average mercury removal efficiency above 90 % at 250 °C. Loading Ag effectively enhanced Hg0 removal activity in high-temperature syngas by generating Ag-Hg alloy via the amalgamation reaction. H2S played the most important role in mercury removal, by adsorbing Hg0 on Ag/Fe-Ti spinel sorbent surface to form active sulfur species. H2S-pretreatment experiments indicated that the reaction of H2S and Hg0 occurred via the Langmuir-Hinshelwood mechanism. Stability and cyclic regeneration experiments indicated that the Ag/Fe-Ti spinel had good regeneration performance and reusability. Density functional theory (DFT) calculation was performed to elucidate that strong chemisorption for H2S and HgS occurred over the Ag/Fe-Ti spinel surface with adsorption energies of −300.35 kJ/mol and −408.12 kJ/mol, respectively. XPS and DFT calculations demonstrated the Hg0 removal mechanism, which the chemisorbed Hg0 reacted with active sulfur species to generated surface-bound HgS. Both XPS and Hg0-TPD analysis certified the presence of HgS and elemental S on the surface of spent Ag/Fe-Ti spinel.