X-ray Absorption Fine Structure Study of the Formation of the Highly Dispersed PdO over ZSM-5 and the Structural Change of Pd Induced by Adsorption of NO

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) were used to investigate the local structure of Pd supported on ZSM-5, which was affected by the acid sites of support and the adsorption of nitrogen oxide. After the thermal treatment of the initially ion-exchanged Pd amine complex, the formation of metal Pd particles was found, whose size was larger than zeolite pore. The subsequent oxidation led to the disruption of Pd particles and the formation of the dispersed PdO, where the degree of dispersion was dependent on the acid amount of ZSM-5. It was also found on Pd/HZSM-5 (Si/Al2 = 24) that the formation of metal Pd and dispersed PdO was reversible upon reduction and oxidation treatments. These facts prove the presence of strong interaction between acid sites and PdO. The role of acid sites of zeolite was considered to keep the dispersed state of PdO. The adsorption of NO on highly dispersed PdO induced a significant change in the local structure of Pd at room temperature. At the same time, PdO was reduced to PdI upon the adsorption of NO. In contrast to Pd/HZSM-5, highly aggregated PdO was found in Pd/NaZSM-5. The deactivation of Pd/HZSM-5 for selective NO−CH4−O2 reaction due to the presence of H2O vapor was considered to be caused by sintering of PdO.