Mechanism Study On Adsorption And Catalytic Oxidation Of Elemental Mercury In Flue Gas On Cerium Oxide Surface

The elemental mercury in coal-fired flue gas is seriously harmful to the environment and human body and is difficult to remove through the existing pollutant control equipments.Therefore,it is of great theoretical and realistic significance to unveil the mechanism of mercury adsorption and catalytic oxidation.CeO2 is a metal oxide with excellent performance,and has good potential for application in the field of mercury removal.However,due to the limitations of the current experimental and characterization methods,the microscopic mechanism of the adsorption and catalytic oxidation of elemental mercury in coal-fired flue gas cannot be fully revealed.In this paper,the quantum chemistry calculation based on the density functional theory is performed to investigate the effect of CeO2 on the adsorption and catalytic oxidation removal of mercury in coal-fired flue gas,the microscopic mechanism of interactions between CeO2 and elemental mercury was explored.Firstly,mercury adsorption mechanism on CeO2-doped carbon-based sorbents was studied,the structure model of carbon-based sorbents composed of multiple benzene rings was established,CeO2 was doped at active sites on the edge of carbon based surface.The calculation results indicate that the adsorption manners of Hg0,HCl,HgCl and HgCl2 on the the carbon-based surface modified by CeO2 are all chemisorption,which illustrates that they are bonded strongly to the adsorbent surface.HCl,HgCl and HgCl2 can all dissociatively adsorb on the carbon surface.After HCl is dissociated,the Cl radical does not bond with the surface.Hg and HgCl in gas phase may combine with Cl radical.The adsorption energy of HgCl and HgCl2 on the surface are relatively high,indicating that the CeO2-doped carbon-based adsorbent has a high efficiency for trapping mercury's oxidation product.The mechanism of catalytic oxidation of elemental mercury in coal-fired flue gas on CeO2 surface was further studied,the mechanism of Hg0 oxidation in the absence of HCl was investigated.A periodic CeO2(111)surface slab model was constructed and the stable adsorption configurations and adsorption energy of Hg0 and Hg0 on CeO2(111)surface were calculated.The reaction pathway of Hg0 oxidation on CeO2(111)surface was conducted.The intermediates involved in the reaction process,the geometry of the transition states,and the rate control steps and energy barriers of the reaction paths were calculated.The results show that Hg0 is physically adsorbed on CeO2(111)surface while HgO is bound to the surface in a chemisorption manner.Depending on the adsorption methods of Hg0 and Hg0,three reaction pathways of Hg0 oxidation by oxygen were proposed.Pathway I is the most likely oxidation route on the CeO2(111)surface due to it having the lowest energy barrier.The formation of the HgO molecule is the rate-determining step,which follows the Mars-Maessen mechanism.By comparative analysis of the reaction energy barriers,it was found that the CeO2 surface has a lower catalytic reaction energy barrier,hence it is a highly efficient elemental mercury oxidation catalyst and has a good application prospect.