The Study Of NH₃-SCR Active Sites At Low-temperature And Hydrothermal Stability Over Novel Cu/SAPO-34Catalyst

As the nitrogen oxides (NOx) emissions increasingly serious today, Urea selectivecatalytic reduction of nitrogen oxides (Urea-SCR) technology has become one of themost important approaches of NOxelimination. In recent years, copper basedchabazite (CHA) type molecular sieves with its excellent NOxselective reductionability, hydrothermal durability, and carbon deposition resistance, become a newgeneration of diesel Urea-SCR catalysts. For the materials research has just started, itis still lack of a comprehensive and systematic understanding for people about itsSCR reaction mechanism and the hydrothermal stabilities. In this study, itconcentrates on exploring the active sites over the Cu/SAPO-34catalyst. Based on theresearch, its hydrothermal stability has been investigated not at high temperatureregions but also at low temperature regions. Try to optimize provide theoretical basisand data support for the application of Cu/SAPO-34molecular sieve in motor vehicleexhaust catalytic reduction application system.On the basic of Cu/SAPO-34catalyst preparation technology, design different Cucontentds which were prepared by ion exchange method. Combined thecharacterization and dynamics methods, it finally figure out that the isolated Cu2+species is the active sites for NH3-SCR reactionBased on the above conclusion, start to explore the changed state of Cu speciesduring the standard water thermal aging process (750oC/12h/10vol.%water vapor).The results showed that the hydrothermal process could be seen as a kind of“activation process”. The thermodynamic state, provided by the process, not onlycontributed to the generation of isolated Cu2+species, but also promoted its internalmigration to the skeleton, and eventually filled with the active sites. Furthermore,only the isolated Cu2+species with the symmetrical structure play the active effectsduring SCR reaction.According to the above conclusions, the hydrothermal stability at both hightemperature and low temperature has been further investigated systematically. At750oC, the Cu/SAPO-34catalysts still remained the excellent SCR reactivity even after120h hydrothermal treatment. Compared with the sample treated at950oC, after12h, the skeleton structure completely deactivated. However, it still maintained theexcellent SCR activity after2h treatment at950oC.It indicates that the induction period of Cu/SAPO-34materials could be shortenas the increasing of hydrothermal temperature. In the basic of the same ageingtreatment period for12h, the low temperature hydrothermal durability has also beenstudied. Results showed that the water vapor advection and penetrate way could notcause the reduction SCR activity. However, after12h immersed in water bath quickly,the crystallization degree and amorphous structure reduced sharply. It indicates thatthe water bath immersion time could lead to acceleration of the dealuminationphenomenon, and result in the isolated state Cu2+species removing with the migrationof Al species, and finally formed the copper aluminate species (CuAl2O3).