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

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/Ce...