Experimental Research On Mercury Catalytic Oxidation Over Ce Doping SCR Catalysts

Mercury is an important environmental pollutant,and most of them exist in elemental form in coal-fired flue gas,which is hardly removed due to its high volatility and low solubility in water.Oxidizing Hg⁰ into Hg²⁺by selective catalytic reduction?SCR?denitrification device and removing through the subsequent flue gas to realize co-benefit of promoting mercury oxidation and NO_x reduction has become an important research scope.In this paper,aiming at the shortcomings of the SCR catalysts for Hg⁰ catalytic oxidation,modified SCR catalysts doped with Ce was prepared by two step impregnation method,which were investigated for the catalytic activity of Hg⁰ and NO_x removal,and the mercury oxidation mechanism.Dependent on this,a new method for regeneration of spent SCR catalysts was proposed.XRD,BET,XPS,TPR,TPD and FTIR methods were employeed to characterize catalysts'surface morphology and structure.First of all,Ce_x/SCR catalysts was prepared by two-step impregnation and 500?calcination and x mean quality score of Ce in catalysts,which ranged from 0 to 5%.The results showed that the CeO₂ and V₂O₅ are highly dispersed on the carrier surface.With the amount of doped Ce increasing,the surface area of Ce_x/SCR catalysts decreased.Ce doping resulted in the reduction of pore volume and pore diameter.Secondly,the effects of temperature,Ce doping amount and flue gas component on the catalytic performance of Ce_x/SCR catalysts were investigated.The results showed that the Ce_x/SCR catalysts had best Hg⁰ oxidation performance at 350?.Ce₁/SCR catalysts performed best at 350?,whose Hg⁰ oxidation efficiency was 21.2%higher than that of SCR catalyst.Ce₁/SCR catalyst exhibited much better resistance to H₂O and slightly weaker resistance to SO₂ than SCR catalyst.NO had an enhancing effect on the Hg0oxidation of Ce₁/SCR catalyst.The NO_x removal efficiency of Ce₁/SCR catalyst were higher than that of SCR catalyst.Thirdly,the Hg⁰ oxidation mechanism over Ce₁/SCR catalyst was investigated.The results showed that O₂ had little influence the Hg⁰ oxidation over Ce₁/SCR catalyst when only O₂ in flue gas.O₂ was the oxidant in Hg⁰ oxidation reaction,but the reaction degeree was low.Activity Cl in HCl was easily combined with Hg²⁺,which can promote oxidation of Hg⁰.Hg⁰ oxidation reaction degree depent on HCl concentration.The catalytic oxidation of Hg0 over the Ce₁/SCR catalyst was consistent with the L-H mechanism.The ratio of chemisorbed oxygen and surface acidity point both were increased by Ce doping.?Hg⁰ was firstly absorbed onto the active sites of Ce₁/SCRcatalyst,which then was oxidized into Hg²⁺by lattice oxygen created in redox cycle of Ce³⁺/Ce⁴⁺and V⁴⁺/V⁵⁺.HCl absorbed on the surface of Ce₁/SCRcatalyst promoted Hg⁰ to oxidize.Finally,the missing lattice oxygen of Ce₁/SCRcatalyst was compensated by O₂ from the flue gas.Finally,spent SCR catalyst was regenerated by complementing V,Mo and doping Ce after cleaning spent SCR catalyst by water washing method.The experimental results showed that Ce content and the efficiency of Hg⁰ oxidation of the modified regenerated catalyst prepared by loading Ce firstly was lower,higher respectively than that by loading V,Mo firstly,while the V content was not affected by load sequence of V,Mo and Ce.As steps of washing and impregnation increased,that toxic chemical content decreased,and surface area and pore volume increased.The highest catalytic oxidation of Hg⁰ was obtained from Ce₂/SCR₁ prepared by adding spent SCR catalysts to 5.4mmol/L Ce?NO₃?₃solution firstly followed by V,Mo loaded,being 27%higher than that of SCR₁ prepared by spent SCR catalysts only V,Mo complemented.The NO_x removal efficiency of Ce₂/SCR₁ catalyst were higher than that of SCR catalyst.Ce_2/SCR₁ catalyst exhibited good resistance to SO₂ due to low Ce doping amout.