We have investigated the CO chemisorption properties of Pd monolayers and ultrathin films of various thicknesses on Ta(110) using TPD, HREELS and LEED. Using Arrhenius plots constructed from CO TPD data, we were able to determine the CO desorption activation energies, Ed, on these films. On the pseudomorphic (θPd= 0.65) and the fcc(111) (θPd= 1.0) Pd monolayers, CO desorbs with nearly the same activation energies, Ed∼ 15 kcal/mol, indicating no significant effects of the changes in the Pd monolayer structure on the CO chemisorption properties. These values are in contrast to Ed= 35.5 kcal/mol measured on the bulk Pd(111) surface and indicate strong alteration in the chemisorption properties of monolayers of Pd on Ta(110). Our CO TPD results also show that the CO chemisorption properties of the Pd(111) surface are practically recovered at θPd= 2.0 if the Pd film is not annealed after deposition at 125 K. For Pd films annealed to 550 K the CO desorption energy increases with the size of the Pd clusters which form on top of the Pd monolayer. HREELS, used to probe the CO adsorption site on these Pd films, shows that CO adsorbs predominantly in atop sites on the pseudomorphic and the fcc(111) Pd monolayers. In addition, a small amount of bridge-bonded CO is detected when the fcc(111) monolayer is exposed to saturation doses of CO. This type of CO corresponds to CO molecules adsorbed on small Pd clusters formed by CO adsorption. This is consistent with our LEED observations which indicate that the fcc(111) structure is reconstructed to the pseudomorphic structure upon exposure to CO. © 1993.
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