Adsorption of CO, H2, O2, and CO2 on clean and Cu-covered Re(0001): an XPS study

The electronic and chemical properties of ultrathin films of Cu on Re(0001) have been investigated by means of X-ray photoelectron spectroscopy (XPS) and chemisorption of CO, H 2 , O 2 and CO 2 . A very similar Cu(2p 3 2 ) binding energy is obtained for a monolayer of Cu on Re(0001) and the surface atoms of Cu(100). Measurements of the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) as a function of film thickness show convergence to bulk properties for films with 4–5 layers of Cu atoms. Chemisorption of CO induces a large decrease in the electron density of the Cu adlayers. This is a consequence of: (1) charge transfer from the Cu overlayer to the Re(0001) substrate (induced by a repulsive interaction between the Cu σ charge and the electrons in the 5σ orbital of CO) and (2) transfer of electrons from the Cu adatoms into the unoccupied 2π orbitais of the CO molecules (π back-bonding). The CuRe interaction enhances the electron donor capabilities of Cu atoms supported on Re(0001), making them more active for CO adsorption and CO 2 dissociation. Hydrogen adatoms shift the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) surfaces toward higher binding energies. The present results indicate that spillover of hydrogen from Re to Cu can occur in mixed Cu/Re catalysts.