The Study On The Removal Mechanisms Of H₂S And Hg⁰in Coal Gas Over Ce-based Oxide

The pyrolysis and gasification of coal is the main technique for coal cleanutilization. H2S and Hg are two important pollutants in gasified coal gas, both ofwhich have negative effects on environment and subsequent chemical process.Therefore, removing H2S in coal gas and reducing the emissions of mercury aremain problems for the clean and efficient use of coal.Ceria has good desulfurization performance and catalytic performance formercury oxidation. Further studies of its desulfurization and mercury removalmechanisms could provide the basic theoretical clues for improving thedesulfurization and mercury removal performances of ceria efficiently in theexperiment. In this study, the density functional theory (DFT) calculation wasused to systemically elucidate the desulfurization mechanism of ceriadesulfurizer. Then, the catalytic oxidation mechanisms of Hg0by Cl-containingspecies on the ceria surface are investigated. Furthermore, the models ofmetal-doped (Mn/Fe) ceria surfaces are constructed to explore the effects ofmetal doping on the oxidation of Hg0and the simultaneous removal of H2S and Hg0. Main conclusions of this study are as follows:The interaction between H2S and ceria shows that H2S adsorbs on thesurface in the partial dissociative adsorption mode. For the formation of SO2, H2and H2O on the perfect ceria surface, the SO2-forming path is the most favorablereaction route based on the kinetics analysis. On the reduced surface, regardlessof whether the dehydrogenation processes or the H2-and H2O-formingprocesses take place, S atom of H2S fills into surface O vacancy and the sulfidesurface is formed.The desulfurization process with ceria desulfurizer mainly contains twosteps. First, surface O vacancy is created to produce the reduced surface. Then Satom is captured by surface O vacancy and CeOS species is formed to achievethe goal of removing H2S. Therefore, the key to improving the desulfurizationefficiency of ceria desulfurizer is that ceria is pre-reduced under the reducingatmosphere to create more surface O vacancies.Cl-containing species could improve the catalytic oxidation performance ofceria to remove Hg0. The path of Cl2dissociation into Cl and then Cl atomsoxidizing Hg0to HgCl2is competed with that of Cl2molecule directly oxidizingHg0to HgCl2in dynamics. HCl can spontaneously dissociate into H and Cl onthe surface, and the dissociation energy is higher than the energy barrier for Hg0oxidation. The adsorbed Cl could spontaneously oxide Hg0to HgCl2. Theoxidation processes of Hg0by Cl-containing species belong to Eley-Ridealmechanism. Surface oxygen vacancy has a negative effect on Hg0oxidation byCl-containing species. HgCl and HgCl2adsorbs on the oxygen vacancy surfacein a dissociative mode. The method of ceria pre-oxidized under oxidizingatmosphere could improve the oxidation efficiency of Hg0by Cl-containingspecies.Surface doping is conducive to removing Hg0oxidized by surface O. Hg0weakly adsorbs on the perfect surface and the coverages of surface adsorbed Odo not have any effect on Hg0adsorption. The activity of surface O is furtherimproved by doping Mn/Fe. The adsorption of Hg0on the doped surfacebelongs to strong chemical adsorption, and HgO or Hg-Mn and Hg-Fe speciesare formed. Both surface lattice O and adsorbed O could oxidize Hg0to formHgO, and the defect surface could be regenerated under O2atmosphere andrecycled.Surface doping is conducive to simultaneously removing Hg0and H2S byceria. HgS adsorbs on the perfect surface in a dissociative mode. The adsorptionand dissociation of H2S are improved by doping Mn/Fe. S atom of H2S couldoxidize Hg0to form HgS, which gets the goal of removing H2S and Hg0simultaneously.