IR studies on the preservation of RhCo3 clusters on the surface of SiO2

The adsorption of RhCo3(CO)(12) on the SiO2 surface, and the interactions of SiO2-suppoaed RhCo3(CO)(12) and SiO2-supported RhCo3(CO)(12)-derived catalyst with CO have been studied at room temperature by IR spectroscopy. A strong physisorption of RhCo3(CO)(12) on partially dehydroxylated SiO2 occurs spontaneously, which results in a dramatic change of carbonyl spectral pattern and downward shifts of both linear and bridged carbonyl bands. The SiO2-grafted RhCo3(CO)(12) is characterized by two carbonyl bands at 2028 and 1830 cm(-1). These observations may be interpreted in terms of a nucleophilic attack of surface oxygen containing groups on the cobalt atoms of the bimetallic cluster. Following decarbonylation of RhCo3(CO)(12)/SiO2 under H-2 at 623 K, the resulting catalyst surface can be recarbonylated by to regenerate the starting bimetallic carbonyl cluster which is then gradually disintegrated to Rh-6(CO)(16) and Co-2(CO)(8) under CO. In parallel, the precursor RhCo3(CO)(12)/SiO2 itself readily reacts with CO to dissociate to Rh-6(CO)(16) and Co-4(CO)(12). The reaction mixture is not only identified on the surface during exposure to CO but also extracted from the surface into the organic solvent. The results suggest that the RhCo, cluster framework is preserved on SiO, despite that the precursor has undergone the severe thermal decarbonylation treatment, and that RhCo3(CO)(12) is regenerated by CO adsorption. A tight SiO, surface-bound RhCo3 cluster is proposed for the supported bimetallic catalyst in terms of cluster-support interaction. In addition, the discrepancy between the regenerated supported RhCo3(CO)(12) and the precursor during the dissociation processes is discussed.