In-situ Pair Distribution Function Study of the Growth of Supported Platinum Particles in Zeolite X
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In-situ Pair Distribution Function Study of the Growth of Supported Platinum Particles in Zeolite X Liliana M. Gámez,1 Oscar Resto2 and Maria M. Martinez-Inesta1 1 University of Puerto Rico-Mayaguez, Chemical Engineering Department PO Box 9000 Mayaguez, PR 00681 U.S.A. 2 University of Puerto Rico-Rio Piedras, Physics Department PO Box 70377 San Juan PR 00931 U.S.A. ABSTRACT Platinum particles supported in zeolites are used as catalysts for hydrogenation/ dehydrogenation reactions. In this study in situ high energy synchrotron X-ray diffraction was used to study the Pt particle formation under calcination and reduction conditions using time resolved pair distribution function (PDF). Because these particles grow inside the pores of zeolite X, PDF is able to give insight to unique information at the short and medium interatomic distance range that cannot be readily obtained with other techniques. Among the information obtained are the Pt atom interactions during calcination and the evolvement of the Pt particle sizes and average Pt-Pt distances during reduction. INTRODUCTION Noble metals are known for their catalytic activity on hydrogenation/dehydrogenation reactions. It is also known that the metal’s surface atoms are those mainly involved in the reaction such that small particles (highly dispersed catalysts) are desired. One way to obtain this industrially is by using a supporting material such as zeolite NaX which has been well studied for the last few decades.1 During a typical synthesis of Pt-NaX catalysts, thermal treatment (from 100 to 350°C ) with oxygen of a Pt(NH3)2+ loaded zeolite using a low heating rate (0.2-1°C/min) has been found to be essential to obtain an homogenous platinum dispersion. During this treatment, different Pt cationic species are formed which are stabilized by the zeolite lattice’s negative charge.2 This results in a close proximity between the zeolite wall and Pt atoms. This oxidation step prevents the sintering process that occurs when the Pt(NH3)2+/zeolite precatalyst is reduced directly with hydrogen.1, 3, 4 The mechanism of nanocluster formation in zeolites is complicated due to the presence of cages, voids and charge compensators that affect their growth and, as such, it is still not completely understood. Moreover, none of the methods used have been able to describe the structural changes occurring during the growth of the clusters during calcination and reduction in the medium length scale (~3-20 Å). This information is necessary to better understand the critical steps during the synthesis of metal particles in the pores of the zeolite. This is in part because of the inherent difficulty in studying nanoscale particles.5 In this work the configurational changes occurring during the synthesis of Pt clusters on the pores of zeolite 13X were studied using the pair distribution function (PDF) technique. This technique was first used to study the structure of the Pt clusters supported in zeolite beta6 where using the Pt differential PDF it was possible to distinguish the Pt-Pt peaks.
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