Modification by lanthanide (La, Ce) promotion of catalytic properties of palladium: Characterization of the catalysts

Pd/gamma-Al2O3 catalysts modified by cerium or lanthanum oxide were prepared according to a standard procedure. The nature of the interaction between the metallic particles and the promoted support was addressed. The effects of changing the following parameters were studied: (i) precursor palladium salt (chloride or nitrate), (ii) content of lanthanide (from the unpromoted catalyst to the pure lanthanide oxide) and (iii) nature of the lanthanide (La or Ce). This paper focuses on the study of the transition metal, whereas the lanthanide redox properties have been reported in an earlier paper (J. Phys. Chem., 1988, 92, 2561). Both structural [transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) at the Pd K edge] and spectroscopic investigations (XPS at the Pd 3d core level) emphasize the role of the precursor palladium salt. When derived from a palladium chloride precursor and after a standard treatment including calcination and reduction, the chloride ion is quantitatively trapped by the support and the metal-support interaction is strong. This is thought to be due to the preferential localization of the chlorine atoms in bridge positions between the metal and the lanthanide. When exposed to an air atmosphere, the palladium atoms at the interface are partially chlorided and cerium is in the +4 oxidation state. After a mild H-2 treatment at 150 degrees C, palladium is fully reduced, except when the cerium content is within 1-5% where the reduction is only partial. Cerium is converted into the trivalent oxidation state either as a chlorinated or an oxychlorinated form. It is thought that this metal-support interaction at the interface, which does not exist in the nitrate precursor catalysts, might explain the catalytic behaviour of such catalysts by the creation of new active sites or a promoting effect (J. Mel. Catal. A, in the press). The strength of this interaction depends on the cation of the support at the interface (Ce3+ > La3+ > Al3+) and is stronger for a cerium content between 1 and 5%.