The Study Of NH₃-SCR Peformance,Hydrothermal Stability And Washcoating Technology Over Cu/SAPO-34 Catalyst By One-pot Method

Since V₂O₅-WO₃-TiO₂ catalysts were introduced into heavy diesel exhaust treatment systems by Daimler AG Company in 2005,the study of mobile source SCR technology was kicked off.In the course of NH₃-SCR catalysts research,the catalysts underwent a revolution from V2O5-WO3-Ti O₂ oxide catalysts to Cu-Fe/ZSM-5 and Fe/Beta zeolite catalyst.Since 2008,the microporous Cu/CHA zeolite catalysts with excellent SCR activity,high N2 selectivity and excellent hydrothermal stability has received widespread attention by domestic and foreign research scholars.In this paper,the Cu/SAPO-34 catalysts were synthesized by one-pot method,namely,the Cu source was added directly to the synthetic material of zeolite molecular sieve.Steady-state and transient-state technique were used to perform the adsorption and reaction behaviors of each reaction atmosphere?NO/NH₃/O₂?at various temperature ranges.At low temperature,O₂ had little effect on the SCR activity;as the temperature increasing,O₂ improved the SCR activity;at high temperature,O₂ inhibited the SCR activity.The NH₃ pulse experiment and NO pulse experiment indicated there existed competitive adsorption between NH₃ and NO at low temperature and competitive reaction between NH₃ oxidation and NH₃-SCR.In view of the difference between silicoaluminophosphate?SAPOs?and aluminosilicates.The hydrothermal stability of Cu/SAPO-34 was investigated from two aspects,namely,low temperature?<100 oC?hydrothermal stability and high temperature?> 750 oC?hydrothermal stability.For the low temperature hydrothermal stability,the destructive effect of H2 O on SAPO-34 framework and Cu²⁺ species protection mechanism at low temperature were studied.A series of Cu/SAPO-34 samples with varing Cu loadings?0-6.78 wt%?were hydrothermally treated at 70 oC with 80% humidity.Textural characterization results showed that this treatment led to the collapse of SAPO-34 framework at zero or low Cu loadings,which was caused by the breakage of Si-O-Al bonds proved by ex-situ DRIFTS and NMR results.Our study indicated that the isolated Cu²⁺ ions were both SCR active site and structure protective agent.For the high temperature hydrothermal stability,the Cu/SAPO-34 catalyst was treated by different temperature and time.XRD results showed that the hydrothermal treatment?950 oC,12 h?could lead to the transformation of CHA structure.NMR results further confirmed that the sample would desiliconize and form Si O₂ and Al PO4 after the high temperature hydrothermal aging.NH₃-TPD results and TPR &EPR results revealed that both the number of Br?nsted acid sites and Cu²⁺ species decreased after the treatment,which likely contributed to the reduced SCR activity.Then the hydrothermal stability of different Cu content,different Si content and different template were investigated.It was found that the samples with high Cu content and high Si content had poor stability and the samples with morpholine and diethylamine as templating agent had superior stability than the samples with triethylamine and tetraethyl ammonium hydroxide as templating agent.Finally,the washcoating technology of the molecular sieve on cordierite moliniths was explored.The influence of coating properties on coated weight was investigated.Washcoat of desired properties were obtained with particle size about 1 ?m and a solid content about 28 wt%.Also the effect of space velocity and coating weight on monolithic catalysts NH₃-SCR activity was studied and an optimized coating process which could solve the low temperature hydrothermal deactivation problem was presented.