Adsorption kinetics and isotopic equilibration of oxygen adsorbed on the Pd(111) surface

Oxygen adsorption on the Pd(111) surface has been studied at 100 and 300 K by temperature programmed desorption (TPD) and isotopic mixing experiments. At 100 K, the sticking coefficient is determined to be 1 up to the coverage of 0.3 O/Pd. The saturation coverage is 0.62 O/Pd, 27% of which dissociates during thermal desorption. Three molecular desorption processes are observed with the activation energy of 7.6, 9.1, and 12.3 kcal/mol, respectively. At 300 K, the sticking coefficient increases with coverage from ∼0.14 at zero coverage to 0.87 at θ≊0.05 O/Pd, then decreases to zero at a saturation coverage of 0.25 O/Pd. The desorption activation energy of 53 kcal/mol is calculated for the associative desorption process with a lateral repulsive interaction of 0.7 kcal/mol. Based on the isotopic mixing results and previous high resolution electron energy loss spectroscopy (HREELS) data, a more complete picture concerning adsorption, conversion, equilibration, desorption, and dissociation processes is suggested.