X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY OF THE ELECTRONIC STATE OF PALLADIUM IN ALKALI-METAL DOPED PD/SIO2 SOLIDS

Alkali metal (Li, Na, K) -promoted Pd/SiO2 catalysts were prepared by adding nitrates to the Pd/SiO2 precursor and subsequent hydrogen reduction. For the alkali metal-free solids the X-ray photoelectron spectroscopy binding energies (3d3/2, 3d5/2) of palladium correspond to metallic palladium. In the presence of alkali metals the binding energies are lowered independently of the possible variations due to particle size. However, it is difficult to distinguish between either a chemical shift, i.e., a change in the electronic state of palladium (via an electron-donating alkali metal-oxygen composite) or a modification of the palladium surroundings and therefore of the relaxation process. The alkali metal is present in an ionic form on the surface of the catalyst. © 1990.