Study on Pd-based bimetallic cluster supported on Y zeolite

Abstract

Applications of supported metal catalysts to industrial processes requires much knowledge about the physicochemical properties of the metal particles. In this study, a comprehensive study on Pd-based bimetallic clusters supported on CaY zeolite has been performed using $^{129}Xe$ NMR spectroscopy, extended X-ray absorption fine structure (EXAFS), xenon adsorption and hydrogen chemisorption measurements. Temperature-programmed desorption (TPD) of hydrogen and catalytic activity of ethane hydrogenolysis have also been studied in order to investigate the effect of the formation of the bimetallic clusters. PtPd and AgPd bimetallic clusters supported on CaY zeolite were prepared by the reduction of sequentially loaded $Pt(NH_3)_4^{2+}$ and $Ag^{+}$ inside a CaY zeolite matrix containing 1-nm Pd clusters. The chemical shift and line width of the $^{129}Xe$ NMR spectrum for the Pd-based bimetallic samples decreased with the addition of Pt or Ag, which indicates the formation of Pd-based bimetallic clusters. However, the $^{129}Xe$ NMR data did not provide information on the structure of the bimetallic clusters. The EXAFS data obtained for the PtPd bimetallic clusters show a significantly large number of the Pt-Pd coordination, confirming the formation of PtPd clusters. However, the Ag-Pd coordination number was difficult to estimate due to similarity the contribution of Ag and Pd to the EXAFS function. Upon addition of the Pt and Ag metals, the total metal-metal coordination numbers of the bimetallic clusters increased, suggesting substantial increases in the cluster size. Such a growth of metal clusters is consistent with decreases in the hydrogen chemisorption and the amount of xenon adsorption on the metal clusters. The TPD spectra of chemisorbed hydrogen on the PtPd bimetallic clusters became very similar to those for 1-nm Pt clusters supported on CaY zeolite as the Pt loading on Pd/CaY increased. Both the EXAFS and TPD data indicate that the surfa...