Journal of Physical Chemistry, Vol.100, No.12, 4996-5003, 1996
Investigation of the Platinum Cluster-Size and Location on Zeolite Kl with Xe-129 NMR, XAFS, and Xenon Adsorption
Although platinum clusters supported on zeolite KL (Pt/KL) were extensively investigated by other laboratories due to remarkable catalytic activity and selectivity for the conversion of linear alkanes to aromatic compounds, there was still some controversy over the cluster size and location in the zeolite channel. The controversy came from difficulty in obtaining high Pt content suitable for the physical characterization without altering the cluster size, compared with the practical catalyst samples. In the present study, we were able to increase the Pt content to 5.2 wt % without changing the physical properties of the Pt/KL, following a procedure using the ion exchange of Pt(NH3)(4)(2+). We have characterized the Pt cluster size and location on the zeolite using the chemical shift in Xe-129 NMR spectroscopy of adsorbed xenon and the X-ray absorption fine structure (XAFS) obtained at the Pt L(III) edge. Results from the Xe-129 NMR and XAFS indicate that the Pt cluster consisted of five to seven Pt atoms located inside the zeolite main channel which is formed by interconnection of cages 1.1 nm in diameter to each other in a linear way through 0.71-nm apertures. The Pt cluster has been found to chemisorb approximately two hydrogen atoms per total Pt at 296 K. The Pt cluster adsorbed as much as 0.4 Xe/Pt at 296 K, which is much more than 0.07 Xe/Pt obtained for a 1-nm Pt cluster entrapped inside the supercage of zeolite NaY (Pt/NaY) under the same conditions. It is believed that a cluster consisting of more than five to seven Pt atoms had difficultly adsorbing such a large quantity of xenon under the experimental condition. The small Pt cluster did not cause considerable pore blockage against the adsorption of Xe (0.43 nm in diameter) and CCl4 (0.59 nm in diameter) into the zeolite pore, indicating the location at a bulged part within the 1.1-nm pore.
Keywords:ABSORPTION FINE-STRUCTURE;NEAR-EDGE-STRUCTURE;HYDROGEN CHEMISORPTION;HEXANE AROMATIZATION;Y-ZEOLITE;SPECTROSCOPY;CATALYSIS;PARTICLE;SURFACE;GROWTH