Identification Of Active Sites And Investigation Of Poisoning Mechanism On Cu-SAPO-34 Denitration Catalyst

Nitrogen oxides(NO_x)are one of the main pollutants in the atmosphere,which are the main cause of acid rain and photochemical smog.NO_x are harmful to the health of people.With the continuous development of transportation industry,the number of diesel vehicles has increased by years.Diesel vehicles have become the main source of NO_x emissions in China.The stringent National VI emission standards are proposed.The selective catalyst reduction(NH₃-SCR)denitrification technology using urea as the reductant is the most efficient and mature technology for the NO_x removal from diesel vehicles.The catalyst is the key of the NH₃-SCR technology.Its performance is the main factor determining the efficiency and economy of the diesel exhaust denitration system.Cu-SAPO-34 zeolite catalyst has received widespread attention due to its excellent low-temperature activity and hydrothermal stability.At present,there is still a lack of in-depth research on the type and distribution of active sites,the relationship between reactivity and structure,and the SCR reaction mechanism over Cu-SAPO-34catalyst.The exhaust gas of diesel vehicles contains SO₂ and alkali metals.The mechanism of their influence on the active sites and structure of the Cu-SAPO-34catalyst is still unclear.Therefore,this paper used theoretical calculation and experimental characterization methods to investigate the type and stable nature of Cu sites in the Cu-SAPO-34 catalyst,the relationship between the nature of active sites and the zeolite framework,and the properties of NH₃ coordinated active Cu sites.Based on the results,the key intermediates and speed control steps in the SCR reaction were explored,and the low-temperature SCR reaction mechanisms were systematically studied.As to the SO₂ and alkali metal poisoning of Cu-SAPO-34 catalyst,the Cu sites sulfation reaction mechanism and activity reduction mechanism were studied.The influence of different alkali metals on the active sites of the catalyst and the alkali metal poisoning mechanism were explored.The relationship between structure and site nature and reactivity was examined.Based on the above research,the main results obtained in this paper are as follows.(1)This study revealed the type,nature and distribution of Cu sites over Cu-SAPO-34 catalyst,and the main active sites in the SCR reactions were identified.Based on the interaction between zeolite framework and Cu species,the nature and structure of different types of Cu sites were obtained using density functional theory calculation method.It was found that the stable positions for Cu sites are six-membered ring window,eight-membered ring window and CHA cage.The chemical potential and Gibbs free energy of different structures were calculated,the stable nature of Cu species under different temperatures and reaction conditions was obtained.It was found that under the condition of 500 K,10%H₂O and 5%O₂,the isolated Cu²⁺site(Z₂Cu)can stably exist in the six-membered ring after H₂O desorption.It is difficult to form stable isolated Cu sites(ZCu and Z₂Cu)in the eight-membered ring,but it is easy to form ZCu OH stable sites.In the case of high Cu loading,Cu dimer structure(Cu-O-Cu)can be formed in the eight-membered ring,and copper oxide(Cu O)clusters can be formed in the CHA cage.H₂-TPR results confirmed the existence of the above-mentioned Cu sites.The adsorption and dissociation energy barriers of NH₃ on different sites were calculated,and it was found that the bivalent Cu sites in the six-membered ring and Cu-O-Cu dimer in the eight-membered ring have the ability to catalyze NH₃dissociation.Besides,the activity tests and H₂-TPR results show that the divalent Cu sites(ZCu OH and Z₂Cu)are the main reactive sites for NH₃-SCR.(2)The migration characteristics of Cu sites under low-temperature NH₃-SCR reaction conditions were discovered,and the types of intermediates corresponding to different forms of Cu sites were revealed.The complete NH₃-SCR reaction mechanism was proposed.With the assistance of NH₃ and H₂O ligands,the Cu site will break the bond with the framework oxygen atoms,and escape from the six-membered ring or eight-membered ring window,exhibiting the ability to migrate.Under typical low-temperature NH₃-SCR reaction conditions,ZCu,ZCu OH and Z₂Cu sites mainly bind NH₃ ligands.Z₂Cu can bind up to three NH₃ ligands to form Z₂Cu-2NH₃/3NH₃,while ZCu and ZCu OH bind two NH₃ ligands to form ZCu-2NH₃and ZCu OH-2NH₃.The energy barrier of NH₃ dissociation at on Z₂Cu-2NH₃/3NH₃ sites is greatly reduced compared to the Z₂Cu site in the framework,which indicates that the migration contributes to the excellent catalytic ability of active Cu sites.The DFT calculation results show that the reaction paths on Z₂Cu-2NH₃/3NH₃ and ZCu OH-2NH₃ sites are different,and two intermediates(NH₂NO and NH₄NO₂)are formed.In addition,the Br?nsted acid sites are favorable for the decomposition of key intermediates.In the re-oxidation process,the formation of a pair of ZCu-2NH₃ complexes reduces the energy barrier of O₂ dissociation,which can promote the re-oxidation reactions.In addition,the participation of H₂O can promote the regeneration of ZCu OH-2NH₃ sites and reduce the production of NH₄NO₂ during re-oxidation process.In the whole reaction cycle,the decomposition of NH₄NO₂ intermediate is the speed control step.Based on the above results,the complete NH₃-SCR reaction mechanism on Cu-SAPO-34 catalyst was obtained.(3)This study revealed the relationship between the SO₂sulfation reaction and the valence and nature of Cu sites.The deactivation mechanism of the catalyst caused by sulfur poisoning was obtained.The experimental results show that Cu-zeolite catalysts are very sensitive to SO₂ poisoning,which are closely related to the reaction atmosphere.After the treatment of Cu-SAPO-34 catalyst by SO₂ at 200?,the NO conversion efficiency dropped by 40%.The NO conversion efficiency dropped by more than 60%in the H₂O-containing atmosphere.DFT calculation and experimental results show that the sulfation reaction mainly occurs during the SCR reaction,causing the sulfation of Cu sites in low valence of+1.The isolated Cu sites with low valence(ZCu and ZCu/ZH)are favorable adsorption sites for SO₂ and O₂,and the reaction energy barriers for the formation of Cu-SO₄ sulfate species are significantly lower than that of the isolated Cu sites(Z₂Cu)with high valence.The sulfation of these Cu sites prevents them from participating in the re-oxidation reaction,which inhibits the regeneration of active Cu sites,causing the activity decrease.In addition,the participation of H₂O reduces the energy barriers for the sulfation reaction on the Cu site with high valence,to generate more stable sulfate species.Thus,a further decrease in activity was caused.The sulfation of ZCu OH sites is proposed to occur when the SO₂ concentration is high,as the adsorption of SO₂ and O₂ with on ZCu OH sites is limited.However,the reaction energy barriers for the formation of sulfate species on ZCu OH site are low.(4)The impact of alkali metals on Cu-SAPO-34 zeolite catalyst was studied,and the irregular poisoning order of three alkali metals(Li,Na and K)was discovered for the first time.The order of poisoning order was Na>K>Li.This is different from the order of basicity corresponding to alkali metals,and also different from the alkali metal poisoning rule on general metal oxide catalysts.The experimental and calculational results show that alkali metals can easily replace H atoms in Br?nsted acid sites,and the substitution capacity is proportional to basicity of alkali metals.The adsorption capacity of NH₃ on alkali metals is much lower than that on Br?nsted acid sites.The reduction of these sites resulted in the abate of the reaction capacity of the catalyst.In addition,alkali metals tend to replace Cu species from active sites to inactive sites to form Cu Al O₂,leading to the decrease of NO conversion rate,due to the reduction of mobility and redox performance of Cu sites.H₂-TPR,EPR and XAFS results confirmed this conclusion.The activity of Na-containing samples decreases more obviously than K-containing samples,because the reaction energy barrier for replacement between alkali metal Na and active Cu species is lower than alkali metal K.Alkali metal K tends to replace H atoms in acid sites.This result shows that the Cu species has great contribution to the SCR reaction activity of the catalyst.The ion diameter of alkali metal Na is close to that of Cu,which is much smaller than that of alkali metal K,resulting in different distribution of Na and K in the zeolite.Based on the above results,it is found that the poisoning effect of alkali metals on Cu-SAPO-34 catalyst is related to the basicity and ion diameter of corresponding alkali metals.