Morphological characterization of zeolite-supported metal catalysts by EXAFS

Abstract

The structure of metal cluster supported on zeolite, alumina and other supports is of fundamental importance in catalysis because it determines its catalytic properties. Extended X-ray absorption fine structure (EXAFS) technique gives the information about the coordination number and distance of the metal element and thus is very useful in the study of the catalyst structure. In this study, morphological characterization of zeolite-supported Pt, Pd and Ru catalysts was performed by EXAFS. All the precursors of metal catalyst were prepared by the ion exchange method. $Pt(NH_3)_4^{2+}$ was ion exchanged into the NaY and CaY zeolites at room temperature (RT). $Pd(NH_3)_4^{2+}$ was ion exchanged into the NaY, CaY and YX zeolites at RT. A Ru-red complex loaded on NaY zeolite was prepared in aqueous solution of ammonia and $RuCl_3$ and 350 K. The precursors of Pt and Pd was calcined in flowing $O_2$ and reduced in flowing $H_2$ to form the clusters. The precursor of Ru cluster was heated under $1×10^{-6}$ kPa and then reduced completely in flowing $H_2$. For X-ray absorption measurements, about 0.1 - 0.2 g of each sample was transformed into a wafer by using a press. These samples were rereduced in flowing $H_2$ in a flow reactor which was jointed with an EXAFS cell. After these samples were transferred into the EXAFS cell, the cell was sealed off by a flame under a $H_2$ atmosphere. X-ray absorption measurement was carried out at RT by using beam line 7C for Pt and 10B for Pd and Ru at the Photon Factory of the National Laboratory for High Energy Physics (KEK-PF) in Tsukuba. Analysis of X-ray absorption data was carried out using a FORTRAN program package by standard procedure. For the Pt clusters supported on NaY and CaY zeolite, it was found from EXAFS data that the coordination numbers were 9.0 and 10.6, respectively, and the Pt-Pt distance was the same as 2.77 . It was obtained from the EXAFS that the coordination number (5.5) of the supported Pd samples was ver...