Extended x-ray absorption fine structure (EXAFS) of highly dispersed rhodium catalysts

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Abstract
An investigation was conducted of the extended x‐ray absorption fine structure (EXAFS) associated with the K‐absorption edge of the rhodium in two catalysts, Rh/SiO2 and Rh/Al2O3, which contained 1 and 0.5 wt. % rhodium, respectively. Hydrogen chemisorption data indicated that the rhodium atoms were predominantly surface atoms in both catalysts. However, the EXAFS data indicated that the average number of nearest neighbor rhodium atoms about a given rhodium atom was much lower for the Rh/Al2O3 catalyst than for the Rh/SiO2 catalyst, 1.5 compared to 9. These values compare with a value of 12 for metallic rhodium. Although the detection of at least some Rh–Rh coordination for the Rh/Al2O3 catalyst indicates that the rhodium is not present exclusively as single atoms bound to the alumina, the EXAFS data do not exclude the possibility that rhodium in this form could be present along with rhodium clusters.