Mobility and clustering of Rh and Ru in zeolite. Experiment and theory

Ruthenium and rhodium have been atomically dispersed on Na zeolite as shown by the infrared spectra of Rh-I(CO)(2) and Ru-I(CO)(2) produced upon reduction in a CO atmosphere. Formation of adsorbed CO bridging two Rh(I) surface atoms upon partial removal of adsorbed CO indicates some Rh(I) species can occupy adjacent surface sites contrary to suppositions in most current literature. While Rh(I) surface species seem to be relatively immobile below 100 degrees C, our data suggest that Rh-0 surface atoms are mobile to some extent. Self-consistent field density functional calculations have been performed for several surface clusters; they indicate that Rh-surface bonds are much weaker for Rh-o species than for Rh(I) species. These calculations support the proposition that Rh-0 surface species are more mobile than Rh(I) surface species.