Migration mechanism on Cu species in CHA-type catalyst for the selective catalytic reduction of NOx during hydrothermal treatment

Abstract

CHA-type aluminosilicate zeolites, have aroused widespread concern recently due to their remarkable physicochemical properties. Meanwhile, Cu-based CHA-type catalysts have been commercialized for desirable low-temperature activity in selective catalytic reduction of NOx. In this work, thermal aging (TA) and hydrothermal aging (HTA) samples were prepared to investigate the respective effects of heat and water on Cu-based CHA-type catalysts during hydrothermal aging treatment. Impressively, TA sample preformed greater SCR activity in low and high temperature range, but HTA sample exhibited a slight decrease compared to the TA sample. The characterization data (obtained by XPS, H2-TPR and in situ DRIFTS techniques) revealed that high temperature promoted the transformation from ZCu(OH) to Z2Cu while high temperature and water facilitated the migration from CuO to Cu2+ during hydrothermal aging. The increase of isolated Cu2+could improve the adsorption capacity of Lewis acid sites at low temperature. However, the gradual drop for the adsorption capacity of Lewis acid sites with the rise of aging temperature indicated the reduction in acid strength after thermal and hydrothermal aging. Additionally, addition of water aggravated the dealumination and reduced the conversion of NOx in the temperature range from 300 °C to 450 °C. While the dealumination occurring at Lewis sites was greatly weakened due to the protection of interaction between Cu and support.