Research On The Hg⁰ Oxidation,NO Reduction,SO₂ Resistance And The Theoretical Calculation Of The Modified Ce/Ti Catalyst In The Flue Gas

Most of the mercury in coal is present in the flue gas as gaseous elemental mercury（Hg⁰）after the combustion process,and it is difficult to be directly removed by the existing air pollution purification facilities.Therefore,the conversion of Hg⁰into easily trapped Hg²⁺has become the focus of current research.Although the SCR unit can oxidize part of Hg⁰ to Hg²⁺,two key factors restrict its mercury removal efficiency:the concentration of HCl in the flue gas and the restriction of the NH₃-SCR reaction.The Ce/Ti catalysts have high efficiency in NO and Hg⁰ oxidation,even in low temperature conditions,but its poor sulfur resistance is restricted its development and application of the system.Fortunately,the rare earth metal modification can improve the performance of Ce/Ti catalyst for NO and Hg⁰ oxidation,and sulfur resistance.Rare-earth metal modified catalysts can cause changes in acid and basic sites to reduce the competition between SO₂ and Hg⁰,improve the SO₂ tolerance and ensure higher oxidation efficiency of NO and Hg⁰.Finally,it is calculated by density functional theory（DFT）to quantitatively explain the above research conclusions,further explore the possible mechanism of catalyst enhanced Hg⁰ removal,and provide data support and theoretical guidance for subsequent rare earth metal modified catalysts.A series of methods were used to prepare Ce/Ti catalysts.Through the research on the performance of the catalyst for the simultaneous catalytic oxidation,combined with the characterization analysis,proved the effect of different preparation methods on the simultaneous NO and Hg⁰ removal of Ce/Ti.The result showed the sol-gel method had the best dispersion effect,maximum specific surface area,more Ce-O functional groups,highly dispersed active components,and more active sites,which are beneficial to the catalytic oxidation.The catalysts synthesized by all methods showed a certain activity of simultaneous catalytic oxidation of Hg⁰ and NO.The SG-Ce/Ti synthesized by the sol-gel method has the highest activity in NO and Hg⁰removal.In order to determine the optimal loading ratio of Ce/Ti catalyst and its oxidation efficiency on Hg⁰ and NO in the absence of HCl,Ce_yTi catalyst was prepared by the sol-gel method,combined with BET,XRD,SEM,XPS to analyze the physical and chemical properties of the catalyst and the simultaneous conversion of Hg⁰ and NO The results showed that excessive cerium loading could agglomerate Ce O₂ on the catalyst surface to form a crystal structure and reduce the specific surface area.When the Ce/Ti mass ratio is 0.3,the best conversion efficiency of NO and Hg⁰can be obtained.NH₃ would competed with Hg⁰ for adsorption of active sites and consumes surface active oxygen,resulting in a significant inhibition of the oxidation efficiency of Hg⁰.O₂ could effectively promote the oxidation of NO and Hg⁰ and replenish the active oxygen consumed by the reaction.SO₂ consumed surface active oxygen and caused Ce O₂ to be converted to Dy₂（SO₄）₃,which lead to catalyst deactivation.H₂O also inhibited the catalytic oxidation of NO and Hg⁰.In order to solve the problem of poor sulfur resistance of Ce/Ti catalysts,rare earth metals were used to modify Ce/Ti catalyst and study the effect on the catalytic oxidation performance and sulfur resistance of the modified catalyst.Activity testing and characterization analysis could draw the following conclusions:the catalyst modified by rare earth metals significantly improved the Hg⁰ and NO oxidation and SO₂ tolerance of the catalyst.The doping of rare earth elements changed the surface structure of the catalyst,inhibited the crystallinity of Ce O₂,promoted the transformation of Ce O₂ from an amorphous structure,increased the specific surface area of the catalyst,which facilitated the oxidation of NO and Hg⁰;the performance of the catalyst is the same as that the redox pair of Ce⁴⁺/Ce³⁺was closely related to the surface oxygen species.The H₂-TPR results showed that the doping of rare earth metals improveed the reducibility of the Ce/Ti catalyst.In addition,the Dy-modified catalyst had the largest specific surface area,more active sites and H₂-TPR reduction peak area,and the performance test results also showed that it had the highest the oxidation effciencies of NO and Hg⁰ and sulfur resistance among these rare earth elements.The poor sulfur resistance of Ce/Ti catalysts has always restricted the development of the catalyst and the rare earth metal modified Ce/Ti catalysts can significantly improve its sulfur resistance.XPS,XRD,BET,TG and other methods were used to analyze the mercury oxidation and sulfur resistance mechanism of the catalyst modified by different metal oxides,combining NH₃-TPD and CO₂-TPD to study the change of acid-base sites on the catalyst surface caused after Dy modification.In the presence of SO₂,the Dy-Ce/Ti catalyst could maintain a high oxidation efficiency for NO and Hg⁰,suggested the sulfur resistance of Ce/Ti catalyst had been significantly improved.According to the characterizations,it was proved that the Dy-doped Ce/Ti catalyst generated Dy₂（SO₄）₃ on the surface of the catalyst,which protected the active component Ce O₂ from the influence of SO₂.The results of NH₃-TPR showed that Dy-Ce/Ti catalyst could not change the total surface acidity of the catalyst,but it increased the number of lewis acid sites which enhanced the oxidation activity of the catalyst.The doping of Dy helped the balance of acidity and redox performance of the Ce/Ti catalyst,thereby inducing better activity.The CO₂-TPD results showed that the Dy-Ce/Ti catalyst had more mid-strong basic sites（Dy-O-Ce）and strong basic sites（Dy₂O₃）.Comparable to weak basic sites,they reacted with acid gas（SO₂）to produce Dy₂（SO₄）₃ or Dy SO₄ preferentially.Due to the migration of Dy³⁺+Ce³⁺（?）Dy²⁺+Ce⁴⁺,Dy-O-Ce preferentially generated Dy₂（SO₄）₃,protecting the active material Ce⁴⁺（?）Ce³⁺oxidation ability.In addition,combined with the characterization results of XPS,the catalytic oxidation of Hg⁰ on the Dy-Ce/Ti catalyst followed the Mars-Maessen mechanism without HCl.Through density functional theory calculations,the modification mechanism of Ce and Dy doping on the electronic structure of anatase Ti O₂ and the effect on the catalytic performance of Hg⁰ were systematically studied.The calculation results show that Ce doping increased the BET surface area of Ti O₂ due to the formation of mixed Ce Ti O₄ materials;Ce doping replaced the original positions of Ti atoms to form a Ce-O-Ti structure,which directly revealed the characteristics of the XRD peak shift and confirmed the conclusion of XPS that the existence of Ce³⁺lead to charge imbalance and the formation of vacancies and unsaturated chemical bonds.There were a large number of Ce-O-Ti structures on the surface of the/Ti catalyst from the crystal model,which could induce the activity of the catalyst and effectively improve the NO and Hg⁰ oxidation.0.89 electron transfer occurred in the substitution of Ce,which confirmed the existence of charge transfer on the Ce/Ti catalyst surface.The doping of Dy also replaceed the position of the original Ti atoms to form a more complex crystal structure,which made the specific surface area greatly increased.In addition,the new appearance of Dy-O-Ti and Dy-O-Ce in the crystal structure structure proves the change of the acid-base sites of the catalyst.At the same time,the inference of XPS,Dy³⁺transfers electrons to NO and Hg⁰ through Ce³⁺（?）Ce⁴⁺,was confirmed.The newly produced structure preferentially generated Dy₂（SO₄）₃,protected the active material Ce⁴⁺（?）Ce³⁺oxidation ability,and also improved the resistance to SO₂ performance.The reasons for the changes in the acid-base sites of the catalyst were explained from the perspective of mechanism calculation.According to the basic process of the Mars-Maessen mechanism,Hg⁰（?）Hg⁰（ads）is the first step in the catalytic oxidation of Hg⁰,which is the rate control step of the entire oxidation reaction.The catalyst loading Dy promoted the physical adsorption of Ce.The reaction rate can increase accordingly.