An EXAFS study of the structure of the metal–support interface in highly dispersed Rh/Al2O3 catalysts

Abstract

Four highly dispersed and fully reduced rhodium on alumina catalysts with different particle sizes in the range 6–12 Å were investigated with the EXAFS technique in order to derive information about the structure of the metal–support interface. This information can only be obtained when the signal‐to‐noise ratio of the experimental EXAFS data is high enough and accurate reference compounds and a modified way of data analysis are used. With the aid of phase and amplitude corrected Fourier transforms it was possible to detect a small additional signal which could be ascribed to a Rh–O bond. Since the catalysts were fully reduced and since the intensity of the small signal increased with decreasing particle size, the oxygen neighbor was assigned to be originated from the metal–support interface. From the intensity of the Rh–O bond it was estimated that, on the average, each interfacial rhodium atom is surrounded by 2–3 oxygen ions of the support. The detected Rh–O bond has a coordination distance of 2.7 Å which is about 0.6 Å larger than the first coordination distance in Rh₂O₃ (2.05 Å). The coordination distance of 2.7 Å can be explained by assuming an interaction between metallic rhodium (atomic radius 1.34 Å) and ionic oxygen belonging to the support (ionic radius 1.4 Å). This would possibly imply an ion‐induced dipole bonding between the metal particle and the support.