Study On Hydrothermal Stability Of CHA Type Molecular Sieve NH₃-SCR Catalyst

Ammonia selective catalytic reduction of nitrogen oxides(NH3-SCR)has been commercially applied to eliminate nitrogen oxides(NOx)from exhaust gases.CuSSZ-13,a CHA-type small pore molecular sieve,has received much attention because of its high SCR catalytic activity.Which,has been practically used in exhaust gas treatment systems for heavy-duty diesel vehicles because of its excellent hydrothermal stability.However,the particle trap(DPF)is arranged before the SCR catalyst,and the regeneration of the particle trap exposes the SCR catalyst to elevated temperature and hydrothermal environment,resulting in hydrothermal aging or even deactivation.The CHA-type molecular sieve Cu-SAPO-34 also has excellent stability against high-temperature hydrothermal aging,but lowtemperature hydrothermal aging occurs in the presence of liquid water and cannot be used in practice.The focus and difficulty of present-day research on molecular sieve-based denitrification catalysts is that the mechanism of hydrothermal aging of molecular sieves is incomplete.The low-temperature hydrothermal aging stability of Cu-SAPO-34 molecular sieve is still to be solved,and there is no effective method to make Cu-SAPO-34 available for practical application without low-temperature hydrothermal aging.In this thesis,an in-depth study of Cu-SSZ-13 molecular sieve catalysts from both the mechanism and improvement of the hydrothermal aging of Cu-SSZ-13 has been conducted.By changing the temperature and atmosphere of Cu-SSZ-13 hydrothermal aging,a series of tests and characterizations were conducted.The results showed that the samples hydrothermally aged under the reducing atmosphere suffered the most severe damage,followed by those hydrothermally aged under inert conditions.In contrast,the samples hydrothermally aged in an ozone-containing atmosphere under elevated temperature conditions supported the highest catalytic activity.This analysis shows that the divalent copper ions in Cu-SSZ-13 under hightemperature hydrothermal conditions undergo self-reduction to the lower valent state.The copper ions in the lower valence state are more likely to move and aggregate,forming copper oxide clusters(CuOx)and causing more severe molecular sieve frameworks collapse and loss of activity.This is the first time that the self-reduction of Cu2+ ions has been noticed in the hydrothermal aging vechanism of Cu-SSZ-13 molecular sieve.On the surface of the Cu-SSZ-13 molecular sieve,an ultrathin SiO2 nanolayer was encapsulated.To improve the elevated temperature hydrothermal aging stability of Cu-SSZ-13 and extend the service life of SCR catalysts for heavy-duty diesel vehicles.Atomic layer deposition(ALD)was used to coat the SiO2 nanolayer.The catalytic activity of Cu-SSZ-13 molecular sieves coated with SiO2 nanospheres was not reduced.After hydrothermal aging treatment,the Cu-SSZ-13 molecular sieve suffered from severe activity loss and skeleton collapse during hydrothermal aging,while the samples coated with SiO2 nanolayer still had good SCR catalytic performance and the skeleton structure was well supported.This ultrathin SiO2 nanolayer can effectively stabilize the surface layer of Cu-SSZ-13,which effectively slows down the damage to the skeleton of Cu-SSZ-13 molecular sieve by hydrothermal aging and improves the stability of Cu-SSZ-13 against hydrothermal aging.The introduction of ALD technology into the preparation of SCR catalysts stabilized the surface layer of molecular sieve,slowed down the damage to the skeleton structure of molecular sieve caused by hydrothermal aging from the surface layer,and supplied a new idea and direction to improve the hydrothermal aging stability of Cu-SSZ-13.Cu-SAPO-34,an SCR catalyst with the same CHA-type small pore molecular sieve,undergoes hydrolysis of skeletal silica in the presence of liquid water below 100℃.An ultrathin TiO2 nanolayer was coated on the surface of Cu-SAPO-34,which covered the end-site silicon atoms and silicon islands on the surface of CuSAPO-34 and did not reduce the SCR catalytic activity of Cu-SAPO-34.After lowtemperature hydrothermal aging treatment,Cu-SAPO-34 loses more SCR catalytic activity and skeleton structure compared to the catalyst covered with TiO2 nanosphere,and the TiO2 nanosphere improves the resistance to low-temperature hydrothermal aging.the hydrophilic groups and silica atoms at the end positions on the surface of Cu-SAPO-34 are covered and form covalent bonds with the TiO2 nanosphere,converting the end positions into non-end positions.This improves the aging problem of Cu-SAPO-34 from a new perspective and gives a new possibility for the commercial application of Cu-SAPO-34.