Mechanistic And Experimental Studies Of Modified CeO₂ Nanorods For The Low Temperature Elemental Mercury Removal With Sulfur Resistance

Mercury and its compounds from coal-fired power plants is another major air pollutant after NO_x,SO₂ and fly ash,and it is the largest source of anthropogenic mercury emissions in China.With the implementation of Minamata Convention,more stringent mercury emission standards will be implemented in China.Existing pollutant control systems for coal-fired power plants can effectively remove mercury species in forms other than elemental mercury（Hg⁰）,which is difficult to capture because of its high volatility and insolubility in water.Catalytic oxidation is a more economical technology to oxidize water-insoluble Hg⁰ to water-soluble oxidized mercury(Hg²⁺),which is then removed by the existing wet desulfurization system.The catalytic oxidation capacity of Hg⁰ is the key role in this technology,and CeO₂ catalysts have received wide attention due to oxygen storage capacity which is benefit for the mercury removal.However,the existing CeO₂-based catalysts are highly dependent on the HCl in flue gas and also poor sulfur and water resistance at low temperatures.Taking above into consideration,different morphologies of CeO₂,supported RuO₂/CeO₂ catalysts and doped Co/Ce catalysts were used as the research systems to investigate the low temperature activity and the reaction mechanism of Hg⁰ oxidation.（1）CeO₂ nanorods with exposed（110）facets and high oxygen vacancy concentration were developed for low-temperature Hg⁰ removal.The characterization results illustrate that CeO₂ nanorods（CeO₂-R）mainly expose（110）facets and CeO₂nanoparticles（CeO₂-P）mainly expose（111）facets.It is found that CeO₂-R with higher oxygen vacancy concentration exhibits higher catalytic activity in the range of 100-300°C,and reaches a maximum mercury removal efficiency of 83%at 200°C.The DFT calculations show that Hg⁰ oxidation barrier on the surface of CeO₂（110）is lower than that on the surface of CeO₂（111）,and the presence of oxygen vacancies facilitates O₂ adsorption,thus generating new active sites for HCl activation.But a higher concentration of oxygen vacancies also leads to the embedding of Cl atoms in oxygen vacancies,leading a higher activation energy to oxidize Hg⁰ compared to the surface adsorbed Cl.The Hg/Cl/O catalytic cycle on the CeO₂ surface can be divided into four parts:（i）HCl activation by lattice oxygen,（ii）Hg⁰ oxidation at the defect surface,（iii）HCl activation by adsorbed oxygen,and（iv）Hg⁰ oxidation at the stoichiometric surface.（2）The complete reaction mechanism of Hg/O₂/SO₂ on CeO₂ catalysts was obtained,and the key steps to enhance the efficiency of Hg⁰ removal under HCl-free conditions on CeO₂ nanorods are enhance the adsorption energy of Hg⁰ and reduce the desorption energy of HgO.It was found that the active temperature of CeO₂ oxidation of Hg⁰ under the condition of O₂ needs to reach more than 300°C,and the rate-determining step is HgO desorption.The order of the interaction between mercury species and the surface follows Hg⁰<HgO<HgSO₄,where Hg⁰ cannot be adsorbed on the surface at the reaction temperature.Combining the results of mechanism analysis and DFT theoretical calculations,the reaction paths of different mechanisms in the Hg/O₂ oxidation reaction were proposed,and the main contribution of Hg⁰ oxidation is from the E-R mechanism.SO₂ in flue gas cannot directly oxidize Hg⁰,and low concentration of SO₂ will slightly promote the oxidation of Hg⁰ by generating HgSO₄with HgO,while high concentration of SO₂ will occupy the active site and consume the active oxygen lead to a decrease in the efficiency of Hg⁰ oxidation.（3）Supported RuO₂/CeO₂ catalysts were investigated for low-temperature Hg⁰removal,and were found to have a stronger adsorption capacity of Hg⁰ adsorption and a lower reaction energy barrier for Hg⁰ oxidation.The results show that the RuO₂/CeO₂-R has more oxygen vacancies than the RuO₂/CeO₂-P,and it exhibited higher Hg⁰ removal efficiency in the range of 100-350°C,and reaching 97.1%at 200°C.Meanwhile,compared with the unloaded CeO₂ nanorods,the sulfur resistance was significantly enhanced.The characterization results revealed that it is mainly the catalyst surface Ce³⁺that are involved in the oxidation of Hg⁰,while the formation of surface sulfate species(SO₄^2-)is the main reason for the influence of SO₂.In addition,Hg⁰ is chemisorbed on the RuO₂ surface and the subsequent oxidation follows the L-H mechanism,and the active oxygen provided by the CeO₂-R carrier can accelerate the oxidation of Hg⁰,indicating the contribution of the carrier oxygen vacancy to the oxidation of Hg⁰ on the RuO₂ surface.（4）Among the metal-doped M/CeO₂（110）（M=Co,Cu,Mn,Ni,Ru,V）surface,Co showed the lowest Hg⁰ adsorption energy and the lowest Hg⁰ oxidation reaction energy barrier,which is favorable for subsequent Hg⁰ removal under O₂ atmosphere.In order to enhance the efficiency of Hg⁰ removal at low temperatures,the Hg⁰ adsorption characteristics of the six transition metals doped CeO₂（110）surface were systematically investigated based on the key steps and reaction mechanism of Hg/O₂ on the CeO₂（110）surface,and the specific pathways of Hg⁰ oxidation were searched to clarify the changes of the reaction transition state structure and its energy barriers.Theoretical calculations show that the Co-doped catalyst has the strongest affinity for Hg⁰ among the six doped catalyst surfaces,with the adsorption energy reaching-47.8 k J/mol,while the Co/CeO₂（110）surface also possesses the highest catalytic activity for Hg⁰ oxidation,and the Hg⁰ oxidation barrier is only 64.1 k J/mol.（5）Co-doped CeO₂ nanorods were investigated as an efficient low-temperature and sulfur-resistant material for mercury removal,and the high concentration of oxygen vacancies could both promote Hg⁰ oxidation and inhibit SO₂ adsorption.The Co doping greatly enhanced the reduction ability and oxygen vacancy concentration of the CeO₂nanorods.The 0.2Co/Ce catalyst possessed the highest activity among all the catalysts,and the efficiency of mercury removal was higher than 95%in the temperature range from 100 to 200°C.Besides,the DFT results confirmed that the presence of oxygen vacancies are benefit for the oxidation of Hg⁰.The 0.2Co/Ce showed excellent sulfur resistance,which was found by the electron transfer between Co and Ce inhibit the adsorption and oxidation of SO₂ on the surface.However,H₂O is easily adsorbed and dissociated on the oxygen vacancy site.On the one hand,the H₂O/OH adsorbed on the active site will inhibit the adsorption of Hg⁰,and on the other hand,the redox cycle of the active site will be broken and the oxidation of Hg⁰ will be inhibited by the inability to continue to produce surface active oxygen.