TPD and XPS study of the CO adsorption on transition-SP metal systems: Pd and Al

Abstract

CO adsorption properties of the bimetallic Pd/Al systems have been studied using thermally programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under UHV conditions. The samples were prepared by vacuum evaporation of Pd onto pure Al substrates. Measured TPD and XPS spectra of CO from bimetallic surfaces were compared to the ones of clean bulk metals. The deposited Pd ad-layers exhibited strong interaction with Al support, which determined the chemical activity of Pd towards interaction with CO. The formation of PdAl alloy of noble metal-like electronic structure with the d-band centered at 4.5eV below Fermi level was observed. The Pd 3d5/2 core level exhibited binding energy 1.8eV higher than that of the surface atoms of bulk Pd with further slight increase finishing at 2.1eV. The thermal desorption spectra of CO from the Pd/Al system consists of two peaks, both lower than those from Pd foil, indicating a distinct weakening in the strength of the CO adsorption bond. Heating induced alloying, an appearance of surface oxygen and a carbon contamination on bimetallic surfaces were observed. The correspondence of these effects with the character of electronic interaction between Pd and Al is discussed.