Characterization of Pd/MgO Catalysts: Role of Organometallic Precursor-Surface Interactions

Pd(acac)2 was supported on a highly dehydroxylated and decarbonated magnesium oxide surface (MgO500), on an untreated MgO surface (MgOAIR), and on a silica. EXAFS and DRIFT data indicate that the organopalladium complex is adsorbed intact on all the surfaces, but its dispersion is dependent on the nature of the surface. On MgO500, dispersion at the molecular level occurs, in contrast to the formation of islands of multilayers on MgOAIR and silica. The final properties of the metal phase, formed by H2 treatment at 500°C, are dependent on precursor-surface interactions. HRTEM analysis shows a narrow distribution of the metal particles centered at 0.6 nm on MgO500, and a wide diameter distribution around 6.1 and 9.9 nm on MgOAIR and SiO2 respectively. In the n-heptane dehydrocyclization at 500°C, the highest selectivity is shown by Pd/MgO500 catalysts and it is consistent with a predominant fraction of very small particles in close contact with surface basic sites.