Regulation of the dispersion of PdO through the interaction with acid sites of zeolite studied by extended X-ray absorption fine structure

The structure of Pd interacted with acid sites of ZSM-5 was studied by using extended X-ray absorption fine structure (EXAFS). Dispersed PdO was generated on ZSM-5 upon the oxidation treatment, where the degree of dispersion was dependent on the acid amount of ZSM-5 support. The reduction of the dispersed PdO with H-2 led to the formation of the agglomerated metal Pd, whose size was larger than ZSM-5 pore. The reoxidation of this metal Pd induced the disruption and the formation of the highly dispersed PdO in ZSM-5 (Si/Al-2 = 24) again, which meant the formation of agglomerated metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. The reversible behavior of Pd was also found on Pd loaded on ZSM-5 with Si/Al-2 ratio of 38 and 55, where the slightly aggregated PdO cluster was observed. The fact evidences the possibility to control the size of PdO simply through changing the acid amount of ZSM-5. The dispersion process of Pd from the agglomerated metal form to the dispersed PdO was followed over Pd/ZSM-5 (Si/Al-2 = 24). The first step was the simple oxidation of the metal Pd to form agglomerated PdO, which occurred between 573 and 673 K. This was followed by the disruption of the agglomerated pho into the dispersed PdO in the temperature range between 673 and 773 K. A novel method to regulate the degree of dispersion through the acid-base interaction between acid sites of zeolite and basic metal oxide was proposed.