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

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. Approximately 10% of adsorbed O₂ irreversibly photodissociated, irrespective of wavelength, with the resulting fragments blocking access to both Fe³⁺ and Fe²⁺ sites for subsequent O₂ adsorption. Preadsorbed water also blocked O₂ adsorption, but postadsorbed water stabilized O₂ at Fe²⁺ sites in TPD to 285 K. The water-stabilized O₂ molecules were insensitive to photodesorption. O₂ photodissociation and photopassivation both represent potentially adverse outcomes in the release of O₂ during the heterogeneous water photooxidation reaction