Alloy formation and CO adsorption on bimetallic Ca/Pd(111) surfaces

Ca films deposited on the surface of Pd(111) were studied by temperature-programmed desorption, X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and low-energy electron diffraction (LEED). Multilayer Ca films desorb at 1180 K, which is much higher than that value for any of the alkali metals. Ca adatoms alloy with Pd at temperatures as low as 700 K. Annealing sub-monolayer Ca films at 1100 K reproducibly formed two well-ordered surface structures with (2 × 2) and (√3× √3)R30° Ca/Pd(111) unit cells depending on the amount of Ca deposited on the Pd(111) substrate. Chemical shifts of Ca XPS peaks indicate that these ordered structures are correlated to the formation of Ca-Pd alloys. The presence of Ca at the Pd(111) surface increases the CO adsorption energy and causes a 110-K shift of the CO thermal desorption peak to a higher temperature than from a clean Pd(111) surface. This Ca-Pd-CO interaction is so strong that it drives Ca out of a Ca-Pd alloyed layer to the overlayer upon CO adsorption, which is evident from the disappearance of the characteristic LEED pattern of the alloy. This migration of Ca atoms occurs without oxidation of Ca. These observations are discussed with reference to possible active phases in Ca-modified, supported Pd catalysts for methanol synthesis and the special nature of Ca over alkali metal promoters for this catalysis.