SURFACE CHARACTERIZATION AND REACTIVITY OF A PD 0.5 MONOLAYER DEPOSITON NI(110)

An upward shift of 0.8 +/- 0.1 eV is measured on the Pd 3d core levels of Pd atoms deposited on Ni(110) with respect to pure Pd surface atom s. These electronic properties are found to be unchanged in the course of a 475 K thermal treatment whereas the activity of the annealed dep osit towards the butadiene hydrogenation reaction is largely enhanced. Consequently, electronic effects are not sufficient to explain the ac tivity enhancement. The LEIS technique reveals that the Pd atoms remai n at the very surface of the sample, the Pd concentration being roughl y 48 at%. Therefore, some geometric effects have to be taken into cons ideration. The annealing would lead to a particular arrangement and wo uld generate more effective catalytic sites.