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Mechanism of Formation of Highly Dispersed Metallic Ruthenium Particles on Ceria Support by Heating and Reduction

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journal contribution
posted on 15.08.2019 by Yosuke Nakaji, Daisuke Kobayashi, Yoshinao Nakagawa, Masazumi Tamura, Kazu Okumura, Keiichi Tomishige
The structural change of Ru/CeO2 and Ru/SiO2 catalysts during pretreatments was investigated by in situ Ru K edge quick-scanning X-ray absorption fine structure (XAFS) combined with temperature-programmed reduction with H2 and X-ray diffraction (XRD). After impregnation and drying, two pretreatment sequences were applied: (i) calcination in air at 573 K and reduction with H2 and (ii) heating in inert gas at 573 K and reduction with H2. In sequence (i) of Ru/CeO2, the extended XAFS (EXAFS) spectra showed the formation of crystalline RuO2 during calcination. On the other hand, in sequence (ii) of Ru/CeO2 and Ru/SiO2, the EXAFS results indicated that the Ru oxide was still noncrystalline after heating. The size of Ru metal particle during reduction was determined by Ru–Ru coordination number in EXAFS (CNRu–Ru) and reduction degree. The size was almost constant for Ru/CeO2 (CNRu–Ru: about 4) or Ru/SiO2 (CNRu–Ru: about 8) in sequence (ii), indicating the absence of aggregation. The size was increased for Ru/CeO2 in sequence (i); however, the XRD results suggested that each RuO2 crystallite was slowly reduced to one Ru particle. After all, aggregation during reduction was not significant for all Ru/CeO2 and Ru/SiO2 catalysts.