Ag/HZSM-5 with Si/Al ratio of 40 was synthesized by the impregnation method. The catalyst materials were characterized by the X-ray diffraction and X-ray absorption spectroscopy by Ag-L3 edge and Ag-K edge. Three different chemical states of Ag (ZO-Ag+, O-Ag+ and metallic Ag, here Z represents Si or Al) in the catalyst. The soot oxidation performance was evaluated by the TG-DTA measurement and showed two combustion stages in tight contact mode which indicates the intrinsic soot oxidation activity. This is because that a lot of Ag ion-exchanged with H ions and existed in the pores of HZSM-5 as ZO-Ag+, which cannot work for soot oxidation. Additionally, this catalyst has a high thermal stability even after high temperature program at 800 °C.
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