The Influence Of Ce On The Low-temperature Hydrothermal Stability Of SAPO-34

The Cu/CHA catalyst with a small pore molecular sieve structure is widely used in the field of NH3-SCR(ammonia selective catalytic reduction).Among them,Cu/SAPO-34 has better resistance to high-temperature hydrothermal capacity and low cost.But at the same time there are fatal problems of low-temperature hydrothermal stability.This paper studies the root cause of the poor hydrothermal stability of SAPO-34 at low temperature,and analyzes the mechanism of low-temperature hydrothermal stability of Ce modified SAPO-34.In this study,Ce ion was introduced into SAPO-34 by one-pot synthesis method and ion-exchange method,and the Ce content in the molecular sieve was evaluated by ICP characterization.The structure and acidity changes of catalysts with different Ce content during LTH aged were systematically studied through XRD,BET,Ex-situ-DRIFTs and other characteristics.Then the effects of Ce ions on the surface acidity and bulk acidity of the catalysts were studied by diethylamine-TPD and NH3-TPD to establishing a relationship between acidity and stability of structural.Then the cerium species of One-pot Ce/SAPO-34 can be studies by H2-TPR,XPS,SEM-EDS mapping and UV-Vis.The result shows that the amount of Ce content is 0.2 wt.%or less,the low-temperature hydrothermal stability of SAPO-34 can be significantly improved.Ce exists in ionic form,it replaces H~+on the Br(?)nsted acidic site of SAPO-34,and generates a Si-O(Ce)-Al structure.And because of its large radius of hydrated ions,it cannot enter the molecular sieve channels and is concentrated on the external surface of SAPO-34.During the LHT(low-temperature hydrothermal treatement),the structural collapse of SAPO-34 begins with the outer layer of Si-OH-Al structure,and then hydrolyzes layer by layer.Ce protects the Br(?)nsted acid site on the external surface of SAPO-34,preventing the hydrolysis reaction,the structure and acidity of the entire SAPO-34 were protected from surface to inside.Then greatly improve the low-temperature hydrothermal stability of SAPO-34.