SCR Catalytic Mechanism And Hydrothermal Stability Of CHA Zeolites:A First-principles Study

With the continuous increase of motor vehicles in China,the vehicle exhaust emission standards are increasingly strict,and the improvement of vehicle exhaust pollution purification technology is imminent.Nitrogen oxides(NOx)are the main components of diesel vehicle exhaust.The ammonia selective catalytic reduction(NH3 SCR)technology can effectively transform the nitrogen oxide in diesel vehicle exhaust into nitrogen and water,which are non-toxic and harmless.CHA zeolites with copper ion exchange(including Cu-SAPO-34 and Cu-SSZ-13)have attracted extensive attention from academia and industry due to the excellent NH3 SCR activity,high nitrogen selectivity and good hydrothermal stability.However,it is quite demanding to investigate the SCR reaction mechanism and the hydrothermal process just by experiment characterations.In this dissertation,we used the first principles calculations to systematically study the NH3 SCR catalytic mechanism in the Cu-CHA zeolites,and explained how water vapor deeply improve the NH3 SCR performance at low temperature.Based on the results,we also investigated the influence of T atoms on the hydrothermal stability in the CHA zeolties.Our research provided a theoretical basis for the synthesis optimization and direction selection of zeolite catalyst for diesel vehicle exhaust purification in China.The main conclusions of the paper are as follows:Firstly,the standard and fast NH3 SCR reactions are discribed in a complete reaction cycle in Cu-SAPO-34 zeolite by a periodic density functional theory(DFT)study.The standard NH3 SCR reaction is a coupling of the activation of NO by O2 and the fast NH3SCR reaction.The Cu(II)sites with copper ions on the six membered ring in Cu-SAPO-34are responsible for the NH3 SCR reaction.According to the reaction mechanism,the NH3SCR reaction can be divided into a reduction by NO+NH3 and an oxidation of the catalyst by NO+O2(NO active cycle)or NO2(fast NH3 SCR cycle),these steps together constitute a complete standard NH3 SCR catalytic cycle.Furthermore,NH3 and NO coadsorb and reaction to form NH₂NO on the same Cu(II)site are required in the reduction part,combined with the Cu(II)be reducted to Cu(I).Finally,the oxidation of Cu(I)site by NO+O2 or NO2leads to the former Cu(II)state of the catalyst.A conclusion of the NH3 SCR reaction mechnism in Cu-SAPO-34 is that all intermediates in fast NH3 SCR are part of the standard NH3 SCR cycle.The detailed reaction paths and reaction barriers calculated by DFT indicate that the oxidation of a NO molecule by a bidentate nitrate ligand to form a NO2 molecule and a nitrite ligand is the rate determining step for standard NH3 SCR.Secondly,first-principles calculations are used to study the adsorption of NH3 on Cu sites and the detailed reaction paths of O2 activation with or without water in Cu-SAPO-34.NH3 adsorption is generally favored over H₂O on Cu ions,yet at low temperature and with partial pressure of water vapor,H₂O and NH3 molecules could co-adsorb on the same Cu site from the phase diagram analysis.During the O2 activation process,this kind of behavior will influence the formation of the Cu(NH3)₂⁺complex and a new kind of Cu(NH3)(H₂O)⁺complex will form during the reduction process of Cu²⁺ions.Compared with O2 activation over pairs of Cu(NH3)₂⁺complexes,Cu(NH3)₂⁺–Cu(NH3)(H₂O)⁺complexes change the torsion angle of the Cu–O–O–Cu configuration and decrease the reaction energy barriers of O2 activation in Cu-SAPO-34.Our results shed light on the water mediated NH3 SCR reaction mechanism of Cu-SAPO-34 at the molecular level.Finally,first-principles calculations are performed to clarify the decomposition mechanism of an H-SSZ-13 framework with adjacent Al atom pair distribution under hydrothermal conditions.It is found that the adjacent Al atoms have a tendency to occupy the parasites of the 4-membered rings in the framework.Water molecules are chemisorbed onto the Al atom one by one,and the hydroxylation of the neighboring O atoms induces the breaking of the Al–O bonds,which causes the first dealumination in 4MRs.The other Al atom in the para-site can be easily removed from the framework once the first one is lost.The feasible subsequent dealumination of adjacent Al atoms would break the linker of 6MRs in the framework,which is responsible for the degraded hydrothermal stability.Moreover,the partial substitution of metal ions(such as Na⁺and Cu⁺)for the protons in the framework will greatly stabilize the Al–O bonds and enlarge the energy barrier of para-site Al dealumination,which leads to the improved hydrothermal stability of H-SSZ-13.