Structure and reactivity of Pt cluster and Pt-based bimetallic cluster supported on KL zeolite

Abstract

Zeolite is a crystalline aluminosilicate containing nanopores with a different size depending on the channel interconnection. The zeolite is a best for encapsulating nanometer size cluster of either metal oxide or metal inside its pore. Recently, KL zeolite has attracted much attention due to its superior activity and selectivity of n-hexane dehydrocyclization, compared to conventional platinum catalysts. The previous works on the Pt cluster in KL zeolite suggests that a 1,6 ring closer of n-hexane occurs preferentially on the cluster surface and the nature of KL zeolite play an important role in the determination of an activity and selectivity. The investigation with X-ray absorption fine structure (XAFS) showed that Pt cluster consisted of ca. 5 - 6 atoms, the smallest cluster size so far obtained inside the zeolite cage. However, a slight increase of the Pt cluster size caused a loss of catalytic activity, which originates from either a high platinum content more than 1 wt% or a sulfur induced cluster growth. In the present work, Pt clusters inside the KL zeolite channel were prepared in a quasi-static manner in order to avoid the pore blockage. The $Pt(NH_3)_4^{2+}$-exchanged precursor was activated in flowing oxygen at 593 K and subsequently reduced in hydrogen flow at 573 K. The resulting sample contained a numerous number of Pt clusters inside the KL zeolite channel without a pore blockage. The results from xenon and $CCl_4$ adsorption measurement, hydrogen chemisorption, XAFS and n-hexane dehydrocyclization showed that most Pt clusters were accessible for n-hexane to convert into benzene. The results of curve fitting of XAFS under controlled atmospheres, hydrogen and helium, suggested that the both intrinsic of platinum and metal-support interaction are responsible for the change of cluster morphology upon hydrogen chemisorption. It is noteworthy that the chemisorption of hydrogen on platinum cluster depleted the hole density from $5d_{5/2}$ ba...