Gámez Mendoza, Liliana

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    Atomic pair distribution study of the growth of Pt nanoparticles in zeolites
    (2016) Gámez Mendoza, Liliana; Martínez Iñesta, María M.; College of Engineering; Cardona Martínez, Nelson; Hernández Maldonado, Arturo; Curet Arana, María C.; Department of Chemical Engineering; Gutierrez, Gustavo
    The Pair Distribution Function (PDF) is a total scattering method used to study the local structure of a material and yields a function in real space with peaks that represent the interatomic distances. In this work, we used this technique to study in situ the particle size changes of growing Pt clusters supported in zeolite X. This method was used because these structures are small and have limited ordering. The Pt particles were refined assuming an fcc structure using two approaches: by using a differential PDF which only includes information of the Pt particles and by using the total PDF which included information of the zeolite and the Pt particles. The results were validated and complemented with results from Scanning Transmission Electron Microscopy, X-ray Diffraction, and Thermogravimetrical Analysis. The differential PDF method was used for the in situ analysis of the two stages during the synthesis of these catalysts: calcination and reduction. It was observed that after the calcination step amorphous particles are formed. Reduction leads to crystalline particles that grow within the support as the temperature increases. Using this technique, we observed how the average particle sizes changed during reduction with temperature and how they organized in an fcc structure. We validated the results with STEM. Using this information, we observed sintering of the nanoparticles from 300C to 350C as a result of outgassing from the Kapton® HN type polyimide capillaries used for these experiments. In this work, we also describe a method to obtain the log-normal particle size distributions (LNSD) from refinement of PDFs. The results were compared directly to experimental distributions obtained by microscopy techniques. It was shown that a number-weighted LNSD obtained from PDF accurately describes the distributions obtained from microscopy. The trend of the particle growth showed a jump in particle size between 300°C and 350°C that is consistent with agglomerative sintering described with STEM. The differential PDF method was used for the in situ analysis of the growth during reduction of Pt nanoparticles using quartz capillaries where the sintering issue was overcome. However, the synthesis of smaller nanoparticles within the support changed the local structure of the zeolite and the reliability of Pt diff-PDFs. In this case a multiphase refinement method was followed to obtain particle size results that were consistent with the particle sizes obtained by STEM. Overall, the PDF is a technique that can be used to model the particle sizes and structures obtained from real catalysts. The analysis of the particle sizes was validated and was consistent with STEM results, suggesting that this technique is reliable. The method can also be used to describe other heterogeneous catalysts with high structural disorder or defects that are not easily analyzed by conventional methods.