Oxygen interactions with the Pt(111) surface

Abstract

Oxygen interaction with the Pt(111) surface has been studied by low energy electron loss (EELS), ultraviolet photoemission (UPS) and thermal desorption (TDS) spectroscopies over the temperature range 100 to 1400 K. Three states of oxygen have been characterized; molecular adsorption predominates below 120 K, adsorbed atomic oxygen predominates in the 150 to 500 K temperature range, while subsurface “oxide” formation may occur in the 1000 to 1200 K temperature range. (Oxide decomposition begins near 1250 K.) The adsorbed molecular state is bound by about 37 kJ/mol to the surface. This weakly adsorbed species has a primary vibrational frequency characteristic of a peroxo species (O 2 −2) indicating an oxygen-oxygen single bond in the adsorbed state. This bonding configuration is also supported by the observed positive change in work function of +0.8 eV and the UPS spectra which indicate that the orbitals derived from the π g ∗ orbitals of molecular oxygen are filled in the adsorbed dioxygen complex. Adsorbed atomic oxygen forms a single adsorbed species on the Pt(111) surface as indicated by the single vibrational frequency observed at 490 cm −1 along with the ordered (2 × 2) LEED pattern. The desorption of atomic oxygen is more complex since the heat of desorption [ d ln(desorption rate) d( 1 RT ) ] decreases from about 500 to 300 kJ/mol with increasing coverages in the low coverage range. In this same coverage region the UPS spectra suggest that non-local electronic effects may be important in determining oxygenplatinum chemistry. Subsurface “oxide” has a single vibrational frequency near 760 cm −1 indicating the formation of a single type of oxide. Thermal decomposition spectra of the oxide also suggest the formation of a simple chemical system since the oxide decomposition curves are smooth above their 1250 K origin.