The structural change of ReO x/SiO 2, Rh/SiO 2, and Rh-ReO x/SiO 2 during the temperature programmed reduction with H 2 was investigated using in situ Re L 3-edge and Rh K-edge quick-scanning X-ray absorption fine structure. Monometallic ReO x/SiO 2 was reduced at about 600 K to form Re metal particles covered with partially oxidized Re species. In contrast, in the case of Rh-ReO x/SiO 2 (Re/Rh = 0.5), the reduction of Rh proceeded to give highly dispersed Rh metal particles at first (∼325 K), and then Re was reduced mildly (∼365 K). The reduced Re species interacted with the Rh metal surface and were highly dispersed. At higher reduction temperature (∼595 K), these Re-modified Rh metal particles aggregated and further reduction of Re to low-valence state proceeded on the Rh metal surface. The low-valence Re interacting with Rh metal surface comprises two-dimensional Re oxide clusters. No detection of Re metal or Rh-Re alloy formation even after reduction at high temperature (595 K) suggests that the two-dimensional ReO x clusters on the Rh metal surface are highly stable.
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