Selective Catalytic Reduction Of NO On Single Atom Doped Transition Metal-based Catalysts: A First-principle Study

With the development of industry and urbanization,the problem of air pollution is becoming more and more serious.Nitrogen oxide（NO_x）is one of the major atmospheric pollutants.It not only causes environmental problems such as acid rain,photochemical smog and ozone layer destruction,but also endangers human health.Automobile exhaust is one of the main sources of NO_x,because the internal combustion engine will generate considerable NO at high temperature.Therefore,the effective elimination of automobles emitted NO is a hot topic in the field of environmental science.Selective catalytic reduction（SCR）is a widely used and effective NO_x elimination technology.In recent years,the strategy of catalytic reduction of NO with CO or H₂ as a reducing agent can produce harmless N₂ and acceptable CO₂ or clean H₂O,so it has attracted much attention.Traditional SCR catalysts are mainly Pt-based catalysts（the active components are Pt,Pd and Rh）.However,the lack of precious metal resources,high cost and poor low-temperature selectivity limit their practical application prospects.Hence,the design of novel SCR catalysts has become a research hotspot in the field of automobile exhaust treatment.According to the previous reports,the construction of transition metal-based alloy or single-atom catalyst（SAC）by single metal atom doping is a promising approach to reduce costs and improve activity.In the present work,based on periodic density functional theory（DFT）calculations combined with microkinetic simulations,a series of transition metal-based catalysts were designed,their reaction mechanisms of selective catalytic reduction of NO were systemically studied,and the effect of doped atoms on activity and selectivity were discussed.The results show that the introduction of the second metal atom can effectively change the electronic structure and coordination environment of the catalyst surface,thereby enhancing the catalytic activity and product selectivity.The main contents and conclusions are summarized as follows:1.NO reduction by CO on transition-metal atom doped platinum surface:a first-principles studyDesigning surface alloy catalysts is one of the most promising ways to improve the NO reduction performance of traditional three-way catalysts.Here,DFT method and microkinetics model were employed to study the NO reduction by CO on the（100）and（111）surfaces of a series of transition metal（TM）-Pt catalysts（TM=Fe,Co,Ni,Ru,Rh,Pd,Os,and Ir）.The stability of the surface alloy catalysts was determined under actual reaction conditions.TM doping improves the adsorption strength of NO.According to the dissociation energy barrier of NO,five TM-Pt（100）（TM=Fe,Co,Ni,Rh,Ir）and three TM-Pt（111）（TM=Fe,Co,Ni）surfaces were screened out to further study the reaction mechanism of NO+CO.All possible elementary steps for the formation of two nitrogen-containing products（N₂,N₂O）and CO₂ were taken into account.Based on the DFT results,microkinetic simulations were carried out.The results show that the conversion rate（TOF）and N₂ selectivity are improved more or less on these TM-Pt alloy catalysts.Fe-Pt（100）,Co-Pt（100）and Ni-Pt（111）have high N₂ conversion rate and almost 100%N₂ selectivity in the whole temperature range of300～1000 K,so they can be considered as good candidates for NO reduction by CO.2.NO reduction by H2 on Ni（110）and Ir/Ni（110）:a first-principles studyThe non-precious metal Ni-based catalysts show good catalytic activity for NO reduction,but the main problem is the high N₂O selectivity at low temperature.In order to improve this problem,we designed an alloy catalyst Ir/Ni（110）by doping the single Ir atom on the surface of Ni（110）.The reaction of NO reduction by H₂ on Ni（110）and Ir/Ni（110）were studied by DFT calculation and microkinetic simulations.The stability of Ir/Ni（110）surface under vacuum and actual reaction conditions is considered.The calculations show on the clean and H-preadsorbed Ni（110）surface,NO is easy to dissociate,and the dissociated N atoms are more likely to couple with the adsorbed NO to form N₂O,rather than with another N atoms to produce N₂.The introduction of Ir atom reduces the formation energy barrier of N₂ and increases that of N₂O.Thus,from the viewpoint of energy barriers,N₂ is easier to be generated on the Ir/Ni（110）surface.Microkinetic analysis further shows that on Ni（110）,the selectivity toward N₂O is 100%below 420 K and that towards N₂ is over 80%above 460 K under ultra-high vacuum conditions,consistent with the experimental results.While on the Ir/Ni（110）surface,the selectivity of N₂ is significantly improved in a wide temperature range（more than90%at 320 K and 100%at T≥340 K）.This work shows that Ir/Ni（110）catalyst can effectively promote the reduction of NO into N₂ and suppress the formation of N₂O.3.NO reduction by CO on Cu2O（110）and Pd1/Cu2O（110）:a first-principles studySelective catalytic reduction of NO by CO is considered to be one of the most effective methods to remove these two pollutants at the same time.Developing low-cost,efficient and stable catalyst is the key to achieve this goal.Cu₂O is widely used in CO oxidation and NO_x elimination,while has high selectivity of N₂O at low temperature.In order to improve the selectivity of Cu₂O to N₂,we designed a single-atom catalyst Pd₁/Cu₂O（110）.In this study,using DFT calculations and microkinetic modeling,we investigated the reaction mechanism of NO reduction by CO on Cu₂O（110）and Pd₁/Cu₂O（110）.The catalytic performance of the two catalysts was compared.It is shown that both surfaces have high CO oxidation activity.The adsorption of NO and CO was strengthened and the preferential configuration of NO on the surface with oxygen vacancies was changed by single-atom Pd doping.The formation pathways of products N₂O and N₂ on both surfaces were considered,and the whole catalytic cycle was determined.Microkinetic analysis showed that the overall conversion rate of NO and the formation rate of CO₂ on the Pd₁/Cu₂O（110）surface are much higher than those on the Cu₂O（110）surface.The N₂O selectivity is 100%at300～450 K on Cu₂O（110）,indicating that N₂O is the only product in the low temperature range,which is consistent with experimental observations.While on the Pd₁/Cu₂O（110）surface,the selectivity of N₂ is 100%in the whole temperature range of300～1000 K.The microkinetic modeling further confirmed that the reaction proceeds through different reaction mechanisms on the surfaces of undoped and doped Pd.The by-product N₂O is formed mainly through intermediate NNO on the Cu₂O（110）surface,while N₂ is formed mainly via the dimer ONNO on the Pd₁/Cu₂O（110）.It is expected that this study can provide useful ideas and clues for the design of oxide-supported single-atom NO reduction catalysts.