Modification of the Catalytic Hydrogenation Activity of Glassy Pd81Si19. Surface Analysis by ISS and XPS

Ion scattering spectra depth profiles and XPS showed that the metallic glass Pd81Si19 is covered in its "as prepared" state with a silicon oxide layer containing little Pd in metallic form, followed in the depth profile by a Si-depleted zone. The bulk composition is reached only at a depth of about 4 nm. The initial alloy contains three phases at the surface: silicon oxide, elemental Si, and elemental Pd but no Pd-Si alloy. Treatments with oxygen at various temperatures up to and above the crystallization temperature and subsequent treatment with reagents dissolving silicon oxide and silicon alter the surface profoundly. The catalytic activity and selectivity can be correlated with the surface composition. The "as prepared" metallic glass was the most selective and with respect to exposed Pd also the most active catalyst, due to the very special distribution of the Pd in the surface. The question of a potential special participation of an amorphous surface structure in the catalytic behavior is discussed.