The influence of crystallite size on the structural stability of Cu/SAPO-34 catalysts

A series of Cu/SAPO-34 catalysts of identical chemical compositions but different crystallite sizes (1-13 mu m) was prepared by one-pot method. The fresh and low-temperature hydrothermally treated (LHT) catalysts were characterized by a comprehensive set of techniques, including XRF, SEM, EDS, XRD, NMR, BET, XPS, pyridine-FTIR, NH3 and diethylamine TPD, EPR as well as H-2-TPR. The reaction rates for NH3-SCR were measured under differential conditions and correlated with their physical and chemical properties. All fresh catalysts, regardless of crystallite size, showed the same bulk properties, such as chemical composition, crystallinity, acidity and copper species. After a hydrothermal treatment (96 h at 70 degrees C with 80% humidity), loss of micropore surface area, micro volume, relative crystallinities, acidity and isolated Cu2+ species was observed on all catalysts, and the extent of decrease was proportional to the average crystallite size. NH3-SCR reaction rate suffered a similar decrease with increasing crystallite size. Surface analyses by SEM-ESD, XPS and diethylamine-TPD showed an obvious surface enrichment of Si (and Bronsted acidity) on all fresh catalysts, and the degree of enrichment is also proportional to crystallite size. This surface enrichment of Si-O-Al linkages is thought to be the root cause for the structural vulnerability found on larger crystallite catalysts under LHT conditions. This understanding is entirely consistent with the observation found on the H/SAPO-34 reference materials of the same sizes, where the structural destruction is even more severe after the LHT. A mechanism for structural disintegration for larger Cu/SAPO-34 crystals was proposed, and the genesis of the Si gradient was discussed in the context of crystallization kinetics.