HIGHLY DISPERSED MGO-SUPPORTED MODEL PD-MO CATALYSTS PREPARED FROM BIMETALLIC CLUSTERS

In attempts to prepare highly dispersed supported palladium catalysts stabilized by molybdenum, an organometallic precursor with Pd-Mo bonds, [Pd2Mo2(Cp)(2)(CO)(6)(PPh(3))(2)], was adsorbed on MgO. The precursor was adsorbed intact, as shown by infrared spectroscopy. For comparison, other samples were prepared from an organopalladium precursor, [PdCl2(PhCN)(2)], and from a mixture of [PdCl2(PhCN)(2)] + [Mo(CO)(6)]. Each supported sample was treated in H-2 at various temperatures to form metallic palladium. The palladium dispersions were characterized by chemisorption of H-2, CO, and O-2; transmission electron microscopy: temperature-programmed desorption of adsorbed CO: and extended X-ray absorption fine structure (EXAFS) spectroscopy, both at the Pd K edge and the Mo K edge. The data show that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form, with the supported clusters being smaller than about 10 Angstrom in average diameter. These clusters have a low capacity for chemisorption of hydrogen and of CO. They are stabilized by the oxophilic molybdenum, which exists preferentially at the interface between the palladium clusters and the metal oxide support. The sample prepared from the two monometallic precursors was characterized by larger palladium particles and by weaker Pd-Mo interactions. The results suggest that the Pd-Mo interactions in the bimetallic precursor were the cause for the high dispersion of palladium in the reduced catalyst.