Characterization of supported cobalt and cobalt-rhodium catalysts : III. Temperature-Programmed Reduction (TPR), Oxidation (TPO), and EXAFS of Co---Rh/SiO2

Temperature-Programmed Reduction, Oxidation, and Extended X-Ray Absorption Fine Structure (TPR, TPO, and EXAFS) experiments supply clear evidence for the formation of bimetallic particles in Co---Rh/SiO2 catalysts. After coimpregnation and drying, as well as after oxidation, the reduction of Co---Rh/SiO2 catalysts proceeds at lower temperatures than the reduction of comparable Co/SiO2 catalysts, indicating that rhodium catalyzes the reduction of the cobalt metal salt and cobalt oxide. EXAFS of the Rh K-edge of the Co---Rh/SiO2 catalyst shows that after reduction the rhodium atoms in the catalyst have less cobalt neighbors than those in the Co---Rh alloy. The Rh---Co and Rh---Rh peak intensities in the Fourier transform of the Rh EXAFS were only slightly influenced by adsorption of oxygen at room temperature, whereas the EXAFS spectrum of the cobalt K-edge changed completely to that of cobalt oxide. From these results it is concluded that the reduced catalyst contains bimetallic Co---Rh particles, the interiors of which are enriched in rhodium, while the outer layers contain more cobalt