| Recently,due to the progressively strict NOx emission standards,reducing NOx in the ambience has turned into a vital research work in the field of environmental catalysis.Selective catalytic reduction of NOx by NH3(NH3-SCR)is the most widely used NOx selective treatment method.The development of efficient catalysts with high selectivity and good catalytic performance has become a key research direction in this field.Quantum chemical calculations can conduct indepth research on the structure of catalysts and reaction mechanism from the micro level,which can provide theoretical basis and design guidance for the development of new catalysts with higher activity and selectivity,and the optimization of reaction process routes.In this paper,the B3LYP method of density functional theory(DFT)was used to conduct a comprehensive theoretical study on the mechanism of NH3-SCR reaction over V2O5/ZSM-5 and V2O5-WO3/ZSM-5,and the influence of WO3 on the reaction.(1)The V2O5/ZSM-5,WO3/ZSM-5 and V2O5-WO3/ZSM-5 catalyst models were constructed,and the adsorption of NH3,NO,NO2 and H2O on the surface of three different catalysts was studied.The adsorption of NH3 is the most stable,which is the initial step of NH3-SCR reaction.The adsorption strength on different catalysts is V2O5-WO3/ZSM-5>V2O5/ZSM-5>WO3/ZSM-5.The addition of WO3 facilitates the transfer of electrons,and significantly increases the adsorption energy of H2O,which is conducive to decompose more Br?nsted acid for the adsorption and storage of NH3.(2)The mechanism of S-SCR reaction on V2O5/ZSM-5 catalyst was studied comprehensively,and the effect of WO3 doping on the reaction mechanism was investigated.S-SCR reaction includes the reduction reaction of reducing NO to N2 and the oxidation reaction with O2 as oxidant,which follows E-R mechanism.In the reduction reaction,Z-[VO2]+ is reduced to Z[VO2H]+ containing B acid,and the rate-determining step is Z-[VO2-NH3]+→Z-[VO2H-NH2]+,and the energy barrier is 134.68 kJ/mol.In the oxidation reaction,Z-[VO2H]+returns to Z-[VO2]+,and the rate-determining step of reduction reaction is Z-[VO2-NH4-O2-NO]+→Z-[VO2-NH4-ONO2]+,and the energy barrier is 32.95 kJ/mol.After doping with WO3,the energy barrier of the reduction reaction over V2O5-WO3/ZSM-5 is reduced to 117.96 kJ/mol,which is conducive to S-SCR reaction.On V2O5-WO3/ZSM-5,the adsorption energy of H2O increases significantly,and B acid decomposed by H2O can be used for the adsorption of NH3.The reaction mechanism is changed,and the rate-determining step is Z-[VOH2WO5-NH3]+-*Z-[VHWO5-NH2-H2O]+,and the energy barrier is 87.11 kJ/mol.So H2O promotes the NH3-SCR reaction.(3)The reaction mechanism of NO catalytic oxidation on V2O5/ZSM-5 is NO(g)→NO(ads)→NO2(ads)+O2(g)→NO2(g)+O2(ads)→O2(ads)+NO(g)→NO2(ads)→ NO2(g),in which the rate-determining step is Z-[VO2-NO]+→Z-[VO-NO2]+,and the energy barrier is 57.02 kJ/mol.In FSCR reaction,NO2 is used as oxidant to directly participate in the reaction to restore Z-[VO2H]+oxidation to the initial state Z-[VO2]+.The ratedetermining step is Z-[VO2H-NO2-NH3]+→Z-[VO2(H2O)-NH2NO]+,and the energy barrier is 24.06 kJ/mol. |
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