Experimental And Theoretical Study Of Cu Ion Exchanged LTA Zeolites For Nitrogen Oxide Removal

Nitrogen oxides（NO_x:NO and NO₂）are the key precursors of fine particulate matters PM_2.5 and ozone,which cause directly and indirectly harm to human health.Selective catalytic reduction with urea and thus ammonia（NH₃-SCR）is a commercially proven technology for mitigating NO_x emissions from diesel exhaust.We have recently shown that copper ions fully exchanged into high-silica LTA-type zeolites（Cu-LTA）are hydrothermally much more stable than Cu-SSZ-13.However,it is still lack of a comprehensive and systematic understanding about the inherent reasons for its superior hydrothermal stabilities,the locations and migration behaviors of the Cu active sites and the active species,as well as the SCR reaction mechanisms over this material in both the presence and the absence of H₂O.In this work we demonstrate that water has abilities to（i）change the type of intrazeolitic copper species,（ii）facilitate the intrazeolitic migration of copper ions,and more importantly（iii）promote the NH₃-SCR activity of the Cu-LTA catalyst.In this work,we located the extra-framework cations in its fresh and aged forms using powder X-ray diffraction（PXRD）and Rietveld analyses.Two different Cu sites were identified in both fresh and aged Cu-LTA catalysts:Cu_sod site is positioned within the sod cage,but is below or above the double 4-membered rings（d4r）unit;Cu_s6r site is located at the single 6-membered ring（s6r）center and the distance from this site to the closest framework O atom is 0.226 nm.DFT calculations reveal that the Cu_sod ions in dry-（Cu-LTA-A_d）and wet-aged（Cu-LTA-A_w）catalysts exist mainly as Cu⁺-s6r_2Aland[Cu（OH）]⁺-s6r_1Al ions,respectively.Structural analysis also reveals that water promotes copper migration from the sod to the lta cage in Cu-LTA,which provides more accessible active sites for NH₃-SCR,thus promotes its catalytic activity.The most important finding of this study is that water promotes NH3-SCR over Cu-LTA at both low-and high-temperatures.We further carried out DFT calculations for the self-reduction behavior of[Cu^II（OH）]⁺and[Cu^II]²⁺species when located on s6r_1Al and s6r_2Alsites,respectively.The DFT calculations suggest that it is both thermodynamically and kinetically more favorable to generate two Cu⁺ions when two[Cu（OH）]⁺ions react around s6r_1Al site.However,the formation of[Cu^II-O-Cu^II]²⁺is more favorable when this reaction happens around s6r2Al sites.This result suggests that H2O benefits the migration of Cu²⁺ions,as well as the formation of side-on?-peroxo（planar）dicopper complex.Furthermore,we elucidated the inherent reasons for the ultra-high hydrothermal stability of Cu-LTA by simulating the dealumination processes using DFT calculation.Moreover,we studied the solvation at 483 K of the[Cu（OH）]⁺ion at the s6r_1Al and the Cu²⁺ion at the s6r_2Al by NH₃,H₂O and O₂,using DFT calculations.Similar to the previously identified[（HO）Cu^II（NH₃）₂)]⁺complex,the[（HO）Cu^II（H₂O）（NH₃）]⁺complex would also form as a reaction intermediate in the presence of water at around s6r_1Al site.However,the competitive adsorption of NH₃ and H₂O molecules should follow[Cu ^II（NH₃）]²⁺→[（NH₄）（HO）Cu^II（NH₃）]²⁺→[（NH₄）（HO）Cu^II（NH₃）₂]²⁺route at around s6r_2Al site.Based on these results,a complete low-temperature NH3-SCR redox cycle was proposed with[Cu^I（H₂O）（NH₃）]⁺as the reaction intermediate.