Pd/Al2O3 interaction: the influence of ionicity character of different alumina surfaces

The nature of the bonding between Pd atoms and alumina surfaces was investigated by analysing the oxygen photoelectron induced Auger KLL transitions. The activity of oxide surfaces was analysed in terms of the surface ability for electronic exchange with the metal adlayer. The comparative study of the metal–substrate interaction on different types of Pd supports (α-Al2O3, γ-Al2O3 and thin alumina layers on polycrystalline Al foil) is presented in this paper. The surface atomic composition of all substrates after cleaned bombardment procedure was determined by ion scattering spectroscopy. The ionicity of the oxygen sites was observed to be a function of the oxide thickness and stoichiometry. It was found that the reduction of the oxide surfaces caused the increase of the I(KL23L23)/I(KL1L23) intensity ratio, i.e. the increase of the ionicity. The opposite tendency for this ratio was observed during Pd deposition on thin alumina oxide. The possible explanations of such behaviour (charge transfer to Pd particle and/or creation of the Al–Pd alloy) are discussed.