CHEMISORPTION OF CL ON RH/SIO2 DETERMINED BY EXAFS

A highly dispersed Rh/SiO2 catalyst was prepared from [Rh(NH3)(5)Cl]Cl-2 by the ion exchange method. Extended X-ray absorption fine structure (EXAFS) measurements showed that after calcination of this catalyst precursor at 573 as well as 673 K each Rh ion had on average slightly more than one Cl ligand. The Rh-O coordination number was higher after calcination at 673 K, probably due to better ordering, and the total Cl + O coordination number was 6. Reduction at 673 K led to small Rh particles containing about 10 metal atoms, as evident from the small Rh-Rh EXAFS coordination numbers and the presence of a Rh-O EXAFS contribution from Rh atoms at the metal-support interface. A Rh-Cl contribution at 2.25 Angstrom showed that during reduction at 673 K not all chlorine atoms had been removed and that the remaining Cl atoms were in contact with Rh atoms. These Cl atoms at the metal surface induced an outward relaxation of the surface Rh atoms, as shown by the Rh-Rh distance which was larger than in bulk Rh.