Mechanism Study On Simultaneous Removal Of NO And SO₂ During Catalytic Oxidation-absorption Process With H₂O₂ Vapor Over Iron-base Catalysts

Coal is the most important energy source in China.Nitrogen oxides(NOx)and sulfur dioxide(SO₂)released during coal combustion can cause serious problems such as haze,acid rain,photochemical pollution,and destruction of the ozone layer,causing damage to human health and the ecological environment.At present,the existing selective catalytic reduction(SCR)technology can not meet the ultra-low NOx emission standards of coal-fired boilers during start-up and shutdown periods and low load conditions due to the limitation of the reaction temperature window(290?450?).In addition,SCR and FGD technologies use a simple serial arrangement,which leads to problems such as large floor space,complex equipment,and high investment and operation costs.In this work,a new method for simultaneous desulfuration and denitration using catalytic oxidation by hydrogen peroxide vapor based on iron-based catalysts coupling with absorption is proposed.The results of experiments and simulated calculations can provide theoretical support for a new approach to achieve simultaneous removal of NOx and SO₂ in a low temperature flue gas.First,?-Fe₂O₃?Fe₃O₄?Fe Mn O?Fe Ti O and Fe Cu O catalysts were prepared and used for simultaneous removal of NO and SO₂.The structure-activity relationship of catalysts were built,and the effects of H₂O₂ concentration,H₂O₂ injection rate,temperature and concentration of simulated flue gas flow rate and flue gas compositions on the simultaneous removal performance were investigated.The results show that Fe₃O₄ has better catalytic oxidation capacity compared than?-Fe₂O₃.Mn,Ti and Cu doping can significantly improve the specific surface area and pore structure of Fe₃O₄,enrich the redox pairs and oxygen vacancy on the surface,and facilitate the formation of·OH and oxidation of NO.Electron paramagnetic resonance(EPR)analysis verified the sequential relationship of·OH content in the catalytic reaction system:Fe Cu O>Fe Ti O>Fe Mn O>Fe₃O₄,which corresponds to denitration efficiency.H₂O₂ concentration of 3 mol/L,H₂O₂ injection rate of30 L/min,reaction temperature of 140? and flue gas flow rate of 1.5 L/min are the optimal operating conditions for the desulfurization and denitration process.SO₂ can be completely removed under different conditions.Density functional theory(DFT)was used to investigate the reaction mechanism between NO and H₂O₂ on the?-Fe₂O₃ catalyst surface.The effect of oxygen vacancy on catalytic oxidation was also discussed.The results show that NO was chemisorbed in molecular form on the perfective and oxygen defective?-Fe₂O₃ surfaces and oxygen vacancy can increase the NO adsorption energy.H₂O₂ adsorption on perfect surface was also in a molecular form;however,H₂O₂ dissociation occurred on oxygen defective?-Fe₂O₃ surface.Oxygen vacancy remarkably enhanced the adsorption intensities of NO and H₂O₂and promoted H₂O₂ decomposition on catalyst surface.Competitive adsorption occurred when NO and H₂O₂ co-adsorbed on the perfective?-Fe₂O₃ surface.As an oxidative product of NO,HNO₂ was synthesized when NO and H₂O₂ co-adsorbed on the oxygen defective?-Fe₂O₃ surface,which confirmed the NO oxidation on the?-Fe₂O₃catalyst surface.Furthermore,the adsorption and decomposition characteristics of H₂O₂ on Fe₃O₄ and Mn,Ti,Cu doped Fe₃O₄ surfaces were studied,and the promotion mechanism of transition metals doping on the H₂O₂ decomposition to hydroxyl radicals was revealed.The reaction pathway of NO oxidation was discovered.The results show that Ti doing on the Fe₃O₄surface could improve the H₂O₂ adsorption energy and directly promote the H₂O₂ catalytic decomposition.However,Mn and Cu doping can promote the generation of oxygen vacancies on the Fe₃O₄ surface and indirectly increase the catalytic activity.NO can be oxidized by hydroxyl radicals generated by H₂O₂ homogeneous decomposition or surface oxygen generated by H₂O₂ heterogeneous decomposition.In order to improve the economy of this simultaneous removal method,Ti O₂ was selected as the catalyst substrate,and a magnetic Fe-based supported catalyst was synthesized by the impregnation-hydrogen reduction method.The effects of relevant operating parameters on the desulfuration and denitration performance of the catalytic oxidation combined with absorption method with H₂O₂ vapor were investigated.The mechanism of catalytic-absorption removal process was analyzed by various characterization methods.The results show that iron oxide was highly dispersed on the Ti O₂ surface,and the Fe element on the catalyst surface mainly existed in the form of divalent iron.Under the optimal reaction conditions,the removal efficiency of NO,NOx and SO₂ were 93.31%,85.90%and 100%,respectively,and the H₂O₂/NO molar ratio was only 1.79.The removal of NO mainly depends on the·OH radicals,while the SO₂ removal mainly depends on the absorption process through Na OH solution.The main products after oxidation and absorption of NO and SO₂ were NO₃^-and SO₄^2-,respectively.Finally,the mechanism of the catalytic oxidation reaction of Ti O₂-supported Fe-based catalysts was revealed by density functional theory calculations.The support and reduction of Fe₂O₃ cluster on the Ti O₂ surface were studied.The adsorption and decomposition properties of H₂O₂ on different surfaces were discussed.The effect of oxygen vacancies,Fe₂O₃ clusters support,and the reduction of Fe₂O₃ clusters on NO adsorption and H₂O₂decomposition were also investigated.The results show that Fe₂O₃ clusters strongly interacted with the Ti O₂ surface and formed a stable system.Oxygen vacancy and reduced iron oxide cluster can promote the adsorption of NO and NO₂,which was beneficial to the catalytic oxidation of NO.Oxygen vacancies,Fe₂O₃ clusters support,and the reduction of Fe₂O₃ clusters on the Ti O₂ surface can improve the H₂O₂ catalytic decomposition.