Targeted Design Of SCR Catalysts And Reaction Mechanism Analysis For Multi-operation Environments

NOx as one of the main air pollutants severely hazards human’s health and ecosystem.Selective catalytic reduction of NOx with ammonia(NH3-SCR)is an effective technology to control NOx discharge in flue gas and catalysts are core of this technology.Recently,as the main emission source of NOx has been transferred from coal-fired power industry to non-power industry and mobile source field,the application of NH3-SCR technology has faced plenty of new problems,such as the lower de-NOx temperature range in non-power industry relative to coal-fired power industry,multipollutant like dioxin and chlorohydrin in flue gas from waste incineration and petrochemicals,and high concentration of water vapor in exhaust of diesel vehicle,which put forward new demands on the adaptability of SCR catalyst.Based on the above background,this paper studies three crucial challenges at present:the development of SCR catalysts adapting to low-temperature operation,the exploration of the mechanism for simultaneously removal of NOx and dioxin on the SCR catalyst,and the development of catalysts with high stability under high-humidity environments,respectively.The main contents and conclusions are as follows.(1)For developing SCR catalysts adapting to low-medium operation,red mud from industrial aluminum smelting as raw material,is treated by selective acid dissolution to extractα-Fe2O3 active components and subsequently modified by Ce element to construct oxidation sites.The intrinsic relationship between redox properties and low-temperature activity was explored by controlling the varieties of Ce precursors.The results show that the temperature windows(NO conversion>90%)of Ce(NO3)3 and Ce(NH4)2(NO3)6 modified red mud(RMcn and RMcan)are extended by 70℃ and 50℃ respectively to low temperature relative to those of only acid dissolution red mud(RM),while the SCR performance of Ce(SO4)2 modified red mud(RMcs)at low temperature deteriorates.Further characterizations indicate the surface Ce species of RMcn and RMcan mainly exist in the form of CeOx and construct Fe-O-Ce bridge bond at the interface which increases the mobility of surface lattice oxygen(O2-),ultimately enhancing the surface oxidation properties.However,the sulfates on the RMcs provide Br(?)nsted acid sites inducing inert adsorbed ammonia formation and poison Fe sites hindering NO adsorption,which cause the E-R and L-H mechanism are suppressed simultaneously during SCR reaction.(2)For exploring reaction mechanism for synergistic removal of dioxins based on SCR catalysts,this paper investigates the effect of chlorobenzene(the model molecule of dioxin)on adsorption behavior and reactivity of SCR reactants and chlorobenzene oxidation process under SCR reaction on the typical V2O5-WO3/TiO2.The results manifest that the Cl species from the dissociation of chlorobenzene combine with V atoms to generate chlorides,which enhance the reactivity of H in the bridging hydroxyl group(V-OH-Ti)and enhanced the surface Br(?)nsted acidity.The aromatic ring after nucleophilic dechlorination combines with the V=O site,weakening the bonding strength of NO chemisorption and inhibiting the active cis-N2O22formation.Additionally,this work proposes a pathway for chlorobenzene catalytic oxidation,and verified the presence of NO and O2 is conducive to CB deep oxidation and NH3 showed negative effect.(3)To design a high-activity SCR catalyst in high-humidity environments,ZSM-5 was firstly treated with selective silicon dissolution to construct an aluminum-rich surface layer with the abundant surface charge density anchoring and functionalizing Cu ion.P modification was subsequently applied to optimize the Cu coordination environment,which promoted the conversion of Cu ions to Cu hydrates with high SCR activity under high-humidity environments.The results exhibit that the selective silicon dissolution exposes the[AlO4]center in the zeolite framework,increasing the Cu ion exchange rate and the active[Cu(OH)]+-Z content.It is determined that 0.2 M was the optimal concentration of the NaOH aqueous solution in the selective silicon dissolution.CZ(0.2)shows the best SCR activity below 300℃,and the temperature window is 75 ℃ wider than that of the untreated catalyst.P and H2O from the highhumidity environment synergistically promote the migration and reconstruction of Cu species to[Cu(OH)(H2O)x]+PO3-hydrate,which reduces the energy barrier of NH3 to NH2 and NH2NO decomposition.Therefore,the NO conversion of CPZ(0.2)in the high humidity environment at 200℃ increased by 27.9%compared with that in the dry environment.This paper provides the construction strategy of low-temperature reactive sites in SCR reaction,the development guidance of bifunctional catalysts for SCR reaction coupled with CB catalytic oxidation,and the regulation strategy of catalysts adapting to high-humidity environments,which have great theoretical and industrial value.