Study on Catalytic Oxidation Mechanism of NO and Hg~0 in Coal-fired Flue Gas by Carbon-based Single Atom Iron Catalysts

Author:Yang Wei Jie

Supervisor:yan wei ping gao zheng yang

Database:Doctor

Degree Year:2019

Download:136

Pages:171

Size:19812K

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Compositions of energy in China are rich coal,poor oil and little gas,coal will dominate energy consumption for a long time.Coal combustion is the main anthropogenic emission source of SO2,NOx and Hg~0 pollutants,which can lead to haze,acid rain,photochemical smog and other environmental problems.Integrated removal of multiple pollutants in coal-fired power plants is the development direction of pollutant control in coal-fired power plants,rapid and efficient catalytic oxidation of NO and Hg~0 is the key to technology route.Now,traditional catalysts have low catalytic activity for NO oxidation and can be easily poisoned and deactivated by SO2 and H2O.To overcome this technical problem,carbon-based single atom iron catalyst was innovatively proposed for catalytic oxidation of NO and Hg~0.In virtue of high activity and selectivity of single atom catalysts,the catalytic activity and anti-poisoning ability can be improved simultaneously.Therefore,the catalytic oxidation mechanisms of NO and Hgoby carbon-based single atom iron catalyst were studied through density functional theory(DFT)calculation and fixed bed experiments.The reaction paths of catalytic oxidation of NO and Hg~0 were revealed,and the effects of reaction conditions on oxidation efficiency of NO and Hg~0 were studied.The effects of arsenic and lead pollutants in coal-fired flue gas on catalytic oxidation of NO and Hg~0 were also discussed.Based on the study of structure-activity relationship in 132 kinds of carbon-based single atom catalsts,a prediction model for adsorption energy of oxygen atoms on carbon-based single atom catalyst was proposed.The research can not only prove the feasibility of carbon-based single atom iron catalyst for catalytic oxidation of NO and Hg~0,but also provide a new direction to solve the problems of low catalytic activity and easily poisoned.First,four typical structures of carbon-based single atom iron catalysts were constructed through defect and nitrogen doping.Four catalyst were single vacancy(Fe/SV-GN),single vacancy with three nitrogen atoms doped(Fe/SV-N3),double vacancy(Fe/DV-GN)and double vacancy with four nitrogen atoms doped(Fe/DV-N4),respectively.Suitable DFT calculation parameters for current systems were selected through the convergence test of cutoff energy and K-point.The interaction mechanisms between Fe atom on single and double vacancy carbon-based supports were revealed through investigation on geometric and electronic structure of four catalysts.Adsorption characteristics of 13 kinds of main flue gas components(N2,O2,CO,NO,HCl,CO2,NO2,SO2,H2O,H2S,N2O,SO3 and NH3)were studied,and the adsorption selectivity for 13 kinds of flue gas on four catalysts were discussed.The adsorption of SO2 and H2O on Fe/DV-N4 belonges to weak physical adsorption,suggesting that Fe/DV-N4 contains anti-poisoning potential for SO2 and H2O.Then,catalytic oxidation mechanism of NO and Hg~0 on the surface of four catalysts were studied based on DFT calculations.Considering O2 and HCl as two oxidants,reaction paths of catalytic oxidation of NO and Hg~0 were investiaged according to the reaction mechanisms of Langmuir-Hinshelwood(L-H),Eley-Rideal(E-R)and Termolecular Eley-Rideal(TER).Configurations of transition states were defined and energy barriers of rate-determining steps were analyzed.Results indicated that O2 can be the oxidant for catalytic oxidation of NO and Hg~0,while HC1 is not suitable as oxidant for catalytic oxidation of Hg~0 for the dissociation adsorption patterns of HgCl and HgCl2 on the surface of catalysts.In the processes of catalytic oxidation of NO and Hg~0,the desorption of product(NO2 and(HgO)2 clusters)is rate-determining step in the whole catalytic reaction.Among four catalysts,Fe/DV-N4 contains the highest catalytic activity for catalytic oxidation of NO and Hg~0,with a 1.26 eV and 2.34 eV of energy barrier in rate-determining steps,respectively.The dominant reaction mechanism is E-R mechanism.Considering the weak adsorption of SO2 and H2O on Fe/DV-N4 and high catalytic activity for catalytic oxidation of NO and Hg~0,Fe/DV-N4 was selected as the research object of catalyst preparation,characterization and catalytic oxidation experiments.Subsequently,FeSA-CN catalyst was prepared by pyrolysis of MOF materials,and advanced characterization methods such as high angle angular dark field-scanning transmission electron microscopy(HAADF-STEM)and X-ray absorption fine structure(XAFS)have proved that the fine structure of prepared FesA-CN is identical to that of DFT calculated Fe/DV-N4.Based on fixed bed experiments,the effects of temperature,reactant concentration,SO2 and H2O on oxidation efficiencies of NO and Hgowere studied.Results indicated that FesA-CN exhibits high catalytic activity,anti-poisoning ability and good stability in catalytic oxidation of NO and Hg~0.The oxidation efficiency of NO can reach 42%under a large gas hourly space velocity(GHSV)of 3.4×105 h-1,NO concentration of 600 ppm,O2 concentration of 6%and a room temperature(25℃).At the temperature range of 25 to 300℃,the oxidation efficiency of NO increases first and then decreases with the increase of temperature,oxidation efficiency of NO reaches the maximum at 150℃.SO2 and H2O have a little promoting effect on catalytic oxidation of NO,while HCl has no effect on catalytic oxidation of NO.The oxidation efficiency of Hg~0 can reach 100%under a large GHSV of 8.5×105 h-1,O2 concentration of 8%,Hg~0 concentration of 120 ug/m3 and a relatively low temperature of 250℃.The reactant concentration and temperature both promote oxidation efficiency of Hg~0.H2O has a little promoting effect on oxidation efficiency of Hg~0,while HCI has a slight inhibitory effect.SO2 has no obvious effect on catalytic oxidation of Hg~0.FesA-CN catalyst has good stability,and the oxidation efficiencies of NO and Hg~0 can be maintained steadily in 60 h stability test.Next,adsorption characteristics of arsenic and lead in flue gas(As2O3,Pb0,PbO and PbCl2)on four kinds of catalysts were studied through DFT calculation,and the effect of arsenic and lead pollutants in coal-fired flue gas on catalytic oxidation of NO and Hg~0 were discussed.Results indicated that adsorption strength order of As2O3,Pb0,PbO,PbCl2 and O2 on Fe/DV-N4 is Pb0>O2>PbO>PbCl2>As2O3 under the temperature range of 300~1000 K.Therefore,Pb0 will inhibit catalytic oxidation of NO and Hg~0,while AS2O3,PbO and PbCl2 could not inhibit catalytic oxidation of NO and Hg~0.The adsorption strength of As2O3,Pb0,PbO and PbCl2 on Fe/SV-N3 is the largest among four catalysts and adsorption type belongs to stable chemisorption.Considering the strong adsorption of Hg~0,HgCl,HgCl2,AS2O3,Pb0,PbO and PbCl2 on Fe/SV-N3 and the wide temperature window,Fe/SV-N3 can be used as a new adsorbent material for simultaneous removal of mercury,arsenic and lead pollutants from flue gas of coal-fired power plants.Last,geometric and electronic structures of 132 kinds of carbon-based single atom catalysts,including 11 kinds of carbon-based supports and 12 kinds of metal atoms,were systematically studied based on DFT calculations.According to binding energy and formation energy of catalysts,the stability map of carbon-based single atom catalyst was plotted.Double vacancy in carbon-based supports have higher stability than single vacancy carbon-based supports.Doping nitrogen atoms can reduce the formation energy of catalysts and facilitate the synthesis of catalysts.Considering the co-contribution of orbital hybridization and charge transfer,a prediction model of oxygen adsorption energy(Eads=0.62X-0.21εd-8.44)was proposed,which consists of d-band center(εd)and system electronegativity(X).The model has a good predictive ability for the adsorption energy of oxygen atoms on 132 kinds of carbon-based single atom catalysts,with a square of correlation coefficient of 0.82.This simple and effective model can accelerate rational design of catalysts and search speed of materials,and provide theoretical basis for further optimizing the catalysts catalytic performance of NO and Hg~0.