Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

Temperature-programmed desorption (TPD) of toluene was carried out on various zeolites, and theoretically analyzed to determine thermodynamic parameters. The TPD process was identified as the case of equilibrium control, showing that the analytical method, the same as that for ammonia TPD, can be applied to the toluene TPD to calculate the adsorption heat. It was observed that approximately one toluene molecule was adsorbed on one Na + cation for MFI and BEA. The heat of toluene adsorption on Na + was MOR > MFI > BEA > FAU, similar to the heat of ammonia adsorption on the corresponding H-form zeolite. A linear relationship was observed between both adsorption heats. It suggests that the adsorption of toluene on Na + is controlled by the electron-withdrawing nature of the ion exchange site, as well as the Brønsted acid strength of the H-form zeolite.

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Pd was loaded on heteropolyacids/SiO2, and used for the selective reduction of NO with methane in the presence of oxygen and moisture. The catalyst exhibited activity in the selective reduction of NO in the temperature range between 473 and 573 K. The temperature was significantly lower than that in the conventional catalysts, such as Pd-loaded zeolites or monolayer-type oxides. Among tested heteropolyacids, H3PW12O40 gave rise to the highest activity in the reaction. The catalytic performance was improved by the addition of water vapor. The generation of highly dispersed PdO species was observed in the Pd K-edge EXAFS spectra, whose structure was similar to those found in the conventional Pd-loaded zeolites catalyst. The reason for the evolution of NO reduction activity over Pd/H3PW12O40/SiO2 was attributed to the formation of NOH+, as measured by IR, in addition to the generation of highly dispersed PdO species at low temperature on the Brønsted acid sites of heteropolyacid.