Photochemical Outcomes of Adsorbed Oxygen: Desorption, Dissociation, and Passivation by Coadsorbed Water

Abstract

A mixed Fe + Cr oxide surface was used to explore the photochemical fate of adsorbed O2 under ultra-high-vacuum conditions. The mixed oxide surface possessed a magnetite-like (111) structure based on low-energy electron diffraction, with its chemical behavior resembling that of Fe3O4(111). Oxygen adsorption at 40 K resulted in two chemisorption states, a strongly bound form desorbing in temperature-programmed desorption (TPD) at 230 K and a weakly bound form evolving at 100 K. The former was assigned to charge transfer adsorption at Fe2+ sites and the latter to electrostatic binding at Fe3+ sites. A minority state was also detected at ∼160 K and tentatively assigned to adsorption at Cr3+ sites. The 230 K O2 state was the focus of photochemical studies employing four wavelengths of light from the red to the UV. Irrespective of wavelength, O2 molecules in the 230 K state preferentially photodesorbed when irradiated, with comparable rates across the visible and an order of magnitude higher in the UV. Approxi...