DFT Calculations Study On Reduction Of Nitrogen Oxides Catalyzed By Single-atom Supported Keggin-type Polyoxometalate

With the continuous development of industry,a large amount of nitrogen oxides(NO_x)are emitted into the atmosphere.They have caused serious environmental problems,and reducing NO_xemissions has become an important issue for mankind.Currently,selective catalytic reduction(SCR)technology using ammonia gas(NH₃)as a reducing agent is the most effective method to remove NO_xfrom stationary and mobile sources.The catalysts used in this technology are V/W/Ti O₂and CHA structure zeolite catalysts,respectively,which have certain shortcomings in stability and catalytic efficiency.Single-atom catalyst(SAC)has the extremely high atomic efficiency,high catalytic activity and good stability,and has become a cutting-edge technology in heterogeneous catalysis field,and has the potential to become a new type of efficient NH₃-SCR catalyst.In this paper,a series of M₁/PTA(M=Mn,Fe,Co,Ni,Ru,Rh,Pd,Ir,and Pt)SACs supported on Keggin-type phosphotungstic acid(PTA)were investigated by density functional theory(DFT)calculations,including the geometry structure,stability,electronic structure,and NH₃-SCR reaction mechanism.Through the calculation of the NH₃-SCR reaction path,the complete reaction mechanism is proposed,and the reaction energy barrier is predicted theoretically,which has certain guiding significance for the rational design of high-efficiency SCR catalysts.(1)Firstly,the geometry structure of M₁/PTA(M=Mn,Fe,Co,Ni,Ru,Rh,Pd,Ir and Pt)SACs with different spin multiplicities was optimized by DFT calculation,and their ground states were determined by energy comparison.By analyzing the M-O bond length and the electron transfer between the anchored single metal atom and the O atom of four-fold hollow(4-H)site,it is shown that the single metal atom can be stably anchored at the 4-H site and exists in the form of oxidation state.The adsorption of different reactant gases for SACs was calculated,including geometric structure and adsorption energy,and it was shown that the 2NH₃-M₁/PTA complex had the highest adsorption energy in different adsorption patterns,except for Ru₁/PTA SAC.The results of spin density and optimization calculations indicate that the active sites of these catalyst are Lewis acid sites rather than Br?nsted acid sites.The frontier molecular orbital(FMO)analysis of the 2NH₃-Rh₁/PTA complex showed that electrons were transferred from the adsorbed NH₃molecule to the anchored single atom,and the mechanism of NH₃molecule activation was proposed.(2)Fast SCR reaction path calculations were performed for Ru₁/PTA,Rh₁/PTA,Pd₁/PTA and Pt₁/PTA,and the complete reaction mechanism was proposed,including the formation and decomposition of two important intermediates of NH₂NO.Coordination-saturated NH₃molecular cannot be directly coupled with NO,and only the activated NH₂fragments can be coupled with NO to form an N-N bond,so activation of NH₃is the most important step in the whole reaction.Analysis of the complete reaction path shows that the four most difficult steps among them are:the first and second NH₃activation,the first and second NHNOH intermediate decomposition,and the rate-determination step is the one with the highest energy barrier among them.The potential energy surfaces of the remaining M₁/PTA(M=Mn,Fe,Co,Ni and Ir)SACs were calculated for these four key steps,and their rate-determination step energy barriers were calculated,indicating that Co₁/PTA has the best SCR performance.The NO oxidation reaction pathway of M₁/PTA(M=Ru,Rh,Pd,and Pt)SACs is calculated,which is proceeded through a novel ER mechanism,and coupling the fast SCR reaction and the NO oxidation reaction can lead to a complete standard SCR reaction pathway.