First Principle Study On Effect Of Catalyst On Working Process And Reduction Products Of Lean NO_x Trap

With the gradual implementation of the"carbon peak"and"carbon neutral"policies,reducing greenhouse gas emissions and achieving a low-carbon transition have become major issues for all industries.The"double carbon"policy has also brought new challenges to the transportation sector and the internal combustion engine industry.The gasoline engine lean combustion technology has received a lot of attention for its low energy consumption,low carbon emission and low emission of conventional pollutants,but in practice,the mixture used in lean gasoline engines deviates from the theoretical air-fuel ratio,so the traditional TWC can not work properly.For the NOx emission control problem of lean gasoline engines,lean NOx trap is gradually gaining more and more attention because of its high NOx conversion efficiency under the exhaust temperature window conditions of conventional gasoline engines and almost no secondary leakage pollution.Currently,most of the studies on the working process of LNT are based on experimental or semi-empirical methods.To address this issue,this paper establishes a Pt-M（M=Pt,Pd,Rh）/γ-Al₂O₃（100）microscopic model of LNT catalyst based on first principle,investigates the effect of noble metal catalysts on the NOx adsorption and reduction process of LNT,and explores the microscopic causes of CO self-inhibition effect.Finally,the mechanism of N₂ and the main by-products of N₂O and NH₃ generation from LNT reduction is analyzed,and the effects of noble metal catalysts on the generation of N₂,N₂O and NH₃ are investigated.Firstly,the effect of noble metal catalyst on the NO adsorption process during LNT adsorption is investigated.The results show that the adsorption strength of NO on the noble metal catalytic sites is influenced by the adsorption configuration and the noble metal catalyst.The adsorption strengths of different NO adsorption configurations are in the order of N-end>Bridge>O-end,and the adsorption strengths of different catalyst configurations are in the order of Pt-Rh>Pt-Pt>Pt-Pd.In terms of optimizing the NOx adsorption process,the NO adsorption intensity should be increased,so the N-end configuration and Pt-Pt and Pt-Rh catalysts are more preferable.The effects of noble metal catalysts on the reduction process of LNT with CO or H₂ as reductant are then investigated.The results show that the adsorption strength of CO is affected by the adsorption configuration and the noble metal catalysts.the adsorption strength of different configurations of CO is C-end>Bridge>O-end,and the adsorption strength of different catalysts for CO is Pt-Pt>Pt-Rh>Pt-Pd.the adsorption strength of H₂is mainly affected by the breakage of H-H bonds and less by the catalysts.The essence of CO self-inhibition effect is that NO and CO adsorption processes at the catalytic sites are similar,and the adsorption of CO occupies the catalytic sites,limiting the adsorption of NO and subsequent processes.From the point of view of optimizing the LNT reduction process,the CO adsorption intensity should be reduced and therefore Pt-Pd catalyst is preferable.Finally,the effect of noble metal catalysts on the formation of LNT reduction products N₂,N₂O and NH₃ is investigated.The results show that the Pt-Pt and Pt-Pd catalysts are weaker for N₂ adsorption,and N₂ is unstable when adsorbed in Bridge configuration.the competitive adsorption between CO and NO will limit the N₂O production,while the greater adsorption strength of N₂O will make the N₂O desorption difficult.Pt-Rh catalyst is the most reducible to NO,and NH₃ molecules are more easily generated on Pt-Rh catalyst;Pt-Pd catalyst is the least reducible to NO,and NH₃ molecules are not easily generated on Pt-Pd catalyst.Pt-Rh catalyst is favorable for NH₃ desorption,and Pt-Pd catalyst is unfavorable for NH₃ desorption.In summary,Pt-Pt and Pt-Pd catalysts and Bridge configuration are favorable for N₂production;Pt-Pd catalyst promotes N₂O production and desorption;Pt-Rh catalyst promotes NH₃ production and desorption,and Pt-Pd catalyst limits NH₃ production and desorption.