Stable Low-Valence ReOx Cluster Attached on Rh Metal Particles Formed by Hydrogen Reduction and Its Formation Mechanism

The structural change of ReOx/SiO2, Rh/SiO2, and Rh ReOx/SiO2 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 ReOx/SiO2 was reduced at about 600 K to form Re metal particles covered with partially oxidized Re species. In contrast, in the case of Rh-ReOx/SiO2 (Re/Rh = 0.5), the reduction of Rh proceeded to give highly dispersed Rh metal particles at first (similar to 325 K), and then Re was reduced mildly (similar to 365 K). The reduced Re species interacted with the Rh metal surface and were highly dispersed. At higher reduction temperature (similar to 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 ReOx clusters on the Rh metal surface are highly stable.