Chemical reactivity of the V–Pd(1 1 1) subsurface alloy: adsorption of CO

The reactivity of a V–Pd(1 1 1)(√3×√3)R30° subsurface alloy has been investigated, using the adsorption of CO as a test case, by photoelectron spectroscopy with synchrotron radiation of the valence and core states and by kinetic analysis of temperature-programmed thermal desorption spectra. The overall conclusion from this study is that the chemical reactivity towards CO is lowered on the subsurface alloy surface as compared to unmodified Pd(1 1 1). The photoemission data indicate that the room-temperature CO saturation coverage is reduced from 0.5 monolayer (ML) on the clean Pd(1 1 1) surface to ∼0.28–0.30 ML on the alloy surface, in excellent agreement with the thermal desorption results. This is explained in terms of a reduced CO adsorption energy, as revealed by the kinetic analysis of the thermal desorption spectra. The balance between threefold hollow and bridge CO adsorption sites at saturation coverage is changed in favour of the preferential occupation of hollow sites on the alloy surface. The observed modifications of the electronic structure of valence states upon alloying and on CO adsorption are well reproduced by a recent ab initio DFT study of CO on V–Pd alloys by Hirschl and Hafner [Surf. Sci. 498 (2002) 37].