An Extended X-ray Absorption Fine Structure Study of Rhodium-Oxygen Bonds in a Highly Dispersed Rhodium/Aluminum Oxide Catalyst

Analysis of in situ EXAFS measurements on a 2.4 wt % Rh/A120, catalyst, reduced at 473 K after calcination at 623 K, shows the presence of two different rhodium-oxygen bonds (viz. 2.05 and 2.68 A). The oxygen neighbors of rhodium at a distance of 2.05 A disappear after reduction at 673 K. The coordination distance 2.05 A is the same as found for Rh203 and indicates that this rhodium-oxygen bond is present due to incomplete reduction of the catalyst at 473 K. Rhodium-oxygen bonds with a coordination distance of 2.68 A are present at both reduction temperatures. These bonds arise from a coordination of the interfacial rhodium atoms with the oxygen ions of the support. Assuming a half-spherical shape for the rhodium metal crystallites, numerical values for the average interfacial rhodium-oxygen coordination number (N = 2.5) and the crystallite diameter (d = 11 A) are derived from the experimental EXAFS parameters