The Comparative Study On Mechanism Of SO₂ Poisoning Over Mn-based And Ce-based Oxides Catalysts For Hg⁰ Removal

The mercury pollution in atmospheric environment is a global problem that has caused widespread concern in the world.Coal-fired power plants are major anthropogenic sources of mercury emissions.Mercury in coal is released in three forms: elemental（Hg⁰）,oxidized（Hg2+）and particulate-bound（Hgp）.Compared with Hg2+ and Hgp,Hg⁰ is very difficult to remove from the flue gas due to its high volatility and low solubility in water.In recent decades,many countries,especially in the developed countries,have taken steps to reduce mercury uses and releases.Recently,the technology for Hg⁰ catalytic oxidation by using the catalyst and O₂ to oxidized mercury which can be desorbed with pyrolysis and collected for reutilization has broad application prospects.However,the Hg⁰ removal efficiency was strongly dependent on many factors,such as preparation method and the presence of SO₂.In this thesis,the opposite Hg removal performance of how can SO₂ promote the catalytic oxidation of Hg⁰ over CeO_x–TiO₂ and inhibit over MnO_x–TiO₂ catalyst was carried out in detail,and the SO₂ poisoning mechanism analysis was analyzed.The physicochemical properties of the catalysts were characterized by BET,XRD,FTIR,Raman,H₂-TPR,TG-DSC,and XPS techniques.The reaction path of SO₂ over the two catalysts was studied.The various influence factors and inherent correlation of promotion and inhibition caused by SO₂ were explored.Furthermore,to prove the mechanism of promotion and inhibition over the two catalysts in the process of Hg⁰ capture,the composition of sediment on the catalyst surface,the evolution and coordination environment of the main elements were also investigated.Moreover,to improve the distribution of surface groups and activity center and obtain higher sulfur poisoning resistance,the modified Mn metal oxide catalyst was prepared by doping transition metal Ru.Through the above research,the important theoretical and scientific basis for in-depth understanding the mechanism of SO₂ poisoning and the important conclusions are summarized as follows:（1）The adsorbent prepared by the sol–gel method showed higher Hg⁰ removal efficiency,which may be owed to the difference of the content and state of MnO_x caused by preparation methods.（2）When the SO₂ was injected,the Hg⁰ removal efficiency over MnO_x–TiO₂ adsorbents decreased dramatically,which can be illustrated that SO₂ can react with manganese oxide to produce their sulfate compounds on the surface of adsorbents.The addition of Ru into MnO_x-TiO₂ adsorbent can not only increase the activity of Hg⁰ removal,but also enhance its SO₂ resistance performance,because the existence of Ru in RMT would preferentially attract SO₂ to produce Ru sulfate and protect the adjacent MnO_x.（3）For CeO_x–TiO₂ adsorbents,the results indicated that in the initial reaction period,the Hg⁰ removal efficiency significantly increased with the presence of SO₂ because of the reaction between CeTi and SO₂.With the reaction time increasing,the promotional effect of SO₂ on Hg⁰ removal over CeTi adsorbent gradually decreased due to the presence of large amount of cerium sulfate species.