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 precurso r was adsorbed intact, as shown by infrared spectroscopy. For comparis on, other samples were prepared from an organopalladium precursor, [Pd Cl2(PhCN)(2)], and from a mixture of [PdCl2(PhCN)(2)] + [Mo(CO)(6)]. E ach supported sample was treated in H-2 at various temperatures to for m metallic palladium. The palladium dispersions were characterized by chemisorption of H-2, CO, and O-2; transmission electron microscopy: t emperature-programmed desorption of adsorbed CO: and extended X-ray ab sorption fine structure (EXAFS) spectroscopy, both at the Pd K edge an d the Mo K edge. The data show that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly disp ersed form, with the supported clusters being smaller than about 10 An gstrom in average diameter. These clusters have a low capacity for che misorption 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 fr om the two monometallic precursors was characterized by larger palladi um particles and by weaker Pd-Mo interactions. The results suggest tha t the Pd-Mo interactions in the bimetallic precursor were the cause fo r the high dispersion of palladium in the reduced catalyst.