Co-adsorption of CO and Pb on Pd(1 1 0)

The adsorption of Pb on Pd(1 1 0) and co-adsorption with CO have been studied by photoemission spectroscopy and low energy electron diffraction. Two ordered structures are formed at sub-monolayer coverages of Pb with symmetry c(2×2) and (3×1). The valence band and Pd 3d core level spectra indicate a strong chemical interaction between Pb and Pd, and the formation of intermetallic bonds with both the first and second layer of Pd. The heat of CO adsorption on the c(2×2) phase is reduced to about half of the value on clean Pd, and even less on the (3×1) structure so that CO does not adsorb at a temperature of 120 K. Structural models are proposed for both of these phases based on the photoemission and co-adsorption results. The results confirm that the de-activation by Pb of Pd as a CO oxidation catalyst is due to the formation of an intermetallic compound, which drastically lowers the adsorption energy. However even the formation of a sub-monolayer phase is sufficient to radically alter the catalytic properties, and the formation of a bulk intermetallic is not necessary.