Synergistic Removal Mechanism Of NH₃-SCR And CO Based On Low Temperature Catalysis

As the emission standards for pollutants such as NOx and CO in steel sintering flue gas are further increased,traditional NH3-SCR（NH3 selective catalytic reduction of NOx）technology has significant shortcomings,especially,when the emission temperature of sintering flue gas is lower than the vanadium-based catalyst window temperature,the catalyst activity is poor and it is easy to form sulfur ammonium salts that can block the catalyst surface pores,resulting in insufficient De NOx catalyst activity.In addition,the removal of CO in sintering flue gas is still in its infancy,and the low-temperature removal efficiency needs to be further improved.Therefore,the development of low-temperature catalysts has become crucial for removing NOx and CO in low-temperature flue gas.This thesis focuses on the excellent CO oxidation,CO--NOx,NH3-NOx characteristics of manganese-based catalysts,and aims to develop low-temperature catalysts with multiple functions.Through the regulation of NH3 and CO on the surface-active sites of the catalyst,the NH3/CO synergistic De NOx reaction is realized.By optimizing the surface structure composition of the catalyst,the high-efficiency synergistic de NOx and ammonia reduction enhancement of NH3 and CO as two reducing agents are achieved in the De NOx process.In this study,single-metal Mn,Co,and Cu catalysts were prepared by precipitation method,and their catalytic activities for NH3-SCR,CO-SCR,and CO oxidation reactions,as well as N2 selectivity,were investigated.The results showed that the Mn catalyst exhibited the best activity for NH3-SCR and CO oxidation reactions,but poor selectivity for N2 and CO-SCR reactions.The Co catalyst showed moderate activity for NH3-SCR and CO oxidation reactions,and no significant decrease in N2 selectivity was observed at higher temperatures.Moreover,in the CO-SCR reaction,the Co catalyst showed better selectivity compared to other samples.In the systematic study of catalytic activity,we found that the single-metal Cu catalyst had the poorest activity compared to other catalysts.A series of Mn-Co oxide catalysts were further prepared using co-precipitation method in this study.The effects of different Mn-Co molar ratios on the sample’s structure,chemical composition,surface state,redox properties,and activity were explored.The mechanism of NH3-CO-SCR reaction was investigated through catalyst surface micro-characterization and reaction pathway deduction.The results indicated that appropriate doping of Mn and Co can effectively enhance catalytic activity and reaction selectivity.In the NH3-SCR activity test of the Mn Co series catalysts,it was found that the MC-3 catalyst with a Mn Co molar ratio of 1:3 exhibited higher catalytic activity,achieving 95%NOx conversion at 100°C.Complete NOx conversion was achieved in the range of 125°C-225°C,and even at 300°C,the conversion rate could still be maintained above 83%.The high-temperature selectivity of NH3-SCR and CO-SCR activity of the MC-X samples were not ideal.However,it was discovered that when a small amount of CO replaced NH3,the NOx conversion rate was basically the same as that of NH3-SCR reaction,and the high-temperature N2 selectivity was effectively improved.Through systematic characterization,it was found that an appropriate Mn-Co ratio can effectively increase the surface Mn⁴⁺content and surface adsorbed oxygen of the catalyst,promote the formation of acid centers,and facilitate the adsorption and activation of reactants.The appropriate redox properties significantly alleviate the non-selective oxidation of NH3 and the direct oxidation reaction of CO,effectively suppressing the generation of N2O and the consumption of reducing agents.Through in situ DRIFTS studies,it was found that the NH3-SCR reaction on the surface of the MC-X catalyst follows the Langmuir-Hinshelwood（L-H）mechanism,while CO mainly participates in the reaction in the gas phase.At low temperatures,the reaction of NO on the catalyst surface suppresses the adsorption of CO.As the temperature increases,the nitrate species react and consume active sites,and some CO adsorbs on the sample surface,which competes with the activation of NH3 adsorption.As a result,the CO oxidation activity of the catalyst decreases in the presence of NH3-SCR,and the oxidation reaction of NH3 also weakens,leading to an increase in N2 selectivity.Finally,based on the excellent catalytic activity of MC-3（Mn Co molar ratio of 1:3）,rare earth element Ce was further doped to obtain better synergistic performance,reduce the amount of NH3 used in the reaction,and improve the high-temperature N2 selectivity.It was found that the introduction of Ce can effectively optimize the microstructure of the Mn Co-based catalyst,promote the occurrence of NH3+CO synergistic catalytic degradation of NO reaction.We further deduced the relevant reaction process through in situ DRIFTS studies and established a NH 3-CO-NO reaction model and reaction mechanism using DFT calculation method.Characterization analysis by XPS,Raman,and other techniques revealed that the optimal doping ratio of Mn:Ce is 1:1.25.At this ratio,Ce atoms can be effectively filled into the Mn-Co solid solution framework,inducing a distortion of the Mn-Co crystal structure.The distortion of the crystal structure is conducive to the precipitation of active ions such as Mn⁴⁺and Co³⁺,further promoting the adsorption and evolution of gas molecules in the reaction.In the NH3+CO+NO reaction system on the surface of the Mn Co Ce catalyst,the reaction between CO and NO is a step-by-step deoxygenation reduction process of coordinated nitrate.As the steps deepen,the reaction energy barrier of the system increases,and the reaction is more difficult to activate,which is the main reason for limiting the activity of the CO-NO reaction.In this system,it was also found that CO reacts with NO3*to generate NO2*,and NO2*reacts with NH4*to generate N2,which produces a relay effect with CO and NH4*.