Adsorption of oxygen on a Pd(111) surface studied by high resolution electron energy loss spectroscopy (EELS)

Oxygen adsorption on a Pd(111) surface has been studied in the temperature range from 30 K to 300 K by high resolution electron energy loss spectroscopy (EELS) and by low energy electron diffraction (LEED). At 30 K oxygen adsorption leads to two roughly equally populated molecular adsorption states characterized by vibrational losses at 850 and 1035 cm-1. These losses are assigned to the O-O stretching frequency of molecularly adsorbed oxygen in a peroxo-like state and a superoxo-like state, respectively. After saturation of these chemisorbed molecular states, a state of physisorbed oxygen with its vibrational frequency close to the gas phase value of 1556 cm-1 is populated. Upon warming the sample above 80 K, an additional loss feature at 650 cm-1 develops which is assigned to a second peroxo-like molecular species. Between 80 and 180 K an interconversion of the different molecular species takes place in which the molecular states are sequentially populated in the order of decreasing vibrational frequency νO-O as the sample temperature is raised. The dissociation process is completed at T ≈ 200 K leaving a layer of atomic oxygen on the surface which is characterized by a vibrational loss at 480 cm-1 and by a 2 × 2 pattern in LEED. The results demonstrate that the dissociative chemisorption of oxygen on Pd(111) does not proceed in a single step but through a sequence of several well defined molecular precursor states. © 1986.