Characterization of zeolite - supported palladium catalyst

Abstract

Palladium catalysts supported on X and Y zeolites of various cationic forms were prepared by using a scheme consisting of $Pd(NH_3)_4^{2+}$ exchange into the zeolite, activation in $O_2$, and subsequent reduction with $H_2$. These catalysts were characterized by $^{129}Xe$ NMR spectroscopy, xenon adsorption method, transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), and hydrogenation of cyclopropane (CP) as a probe reaction. Xenon was found to be adsorbed on the multivalent cations, such as $Ca^{2+}$ and $Y^{3+}$ much more strongly than the zeolite wall or monovalent cations, which was also observed for the Pd cluster. The amount of xenon adsorbed on the Pd cluster was determined from the xenon adsorption isotherm and then used as a sensitive probe for the study of the cluster size variation. The chemical shift in $^{129}Xe$ NMR of the adsorbed zenon gas and TEM were also used to investigate the Pd cluster size dependence on the cation in the zeolite. The results indicate that small Pd cluster of about 1-nm size, Which is stable up to 673K, can be prepared by anchoring on multivalent cations such as $Ca^{2+}$ and $Y^{3+}$ located in the supercage. Otherwise, the cluster grows to the size of above 2 nm upon heating to 673 K. EXAFS experiment was carried at $H_2$ atmosphere and room temperature, and the data were analyzed with the University of Washington package. The EXAFS results for all the supported Pd cluster samples showed that very small clusters were formed with a Pd-Pd bond distance of 0.28 nm and an average coordination number of 5.5. Such a small coordination number, in contrast to ca. 2-nm size from TEM indicated that the Pd cluster occupied a few adjacent supercages. The hydrogenation of CP was carried out over two Pd Catalysts of different cluster sizes, i.e., Pd/NaY and Pb/CaY catalysts, as a test reaction. The reaction rate was measured in a Pyrex recirculation reactor system in the temperature range of 196-25...