Roles of Fe2+, Fe3+, and Cr3+ surface sites in the oxidation of NO on the (Fe,Cr)3O4(1 1 1) surface termination of an α-(Fe,Cr)2O3(0 0 0 1) mixed oxide

• The mixed Fe + Cr corundum (0 0 0 1) oxide film employed possessed a magnetite-like (1 1 1) surface structure.
• Relative adsorption strengths for NO and O2 at surface sites follow the trend: Fe2+ > Cr3+ > Fe3+.
• NO oxidation to nitrate only occurs at Fe2+ sites.
• Molecular oxygen is the active form of oxygen for NO oxidation.
• Visible light irradiation promotes NO photodesorption but not photooxidation.

The oxidation of NO was explored on a mixed Fe + Cr oxide surface using temperature-programmed desorption (TPD). NO desorbs from (Fe,Cr)3O4(1 1 1) in two main peaks at 220 and 370 K, with a third minor peak at ∼315 K. O2 TPD shows similar behavior. The strongly and weakly bound molecules are due to adsorption at Fe2+ and Fe3+ sites, respectively, and the minor states are assigned to Cr3+ sites. No thermal decomposition was detected for adsorbed NO, whereas ∼10% of the adsorbed O2 dissociated at Fe2+ sites. NO reacts with preadsorbed O2 to produce surface nitrate, as confirmed by isotopic labeling, which decomposes in TPD at 425 K. Atomically adsorbed O does not react with NO. Fe3+ and Cr3+ sites do not appear to participate in NO oxidation. Irradiation of adsorbed NO or NO + O2 with 460 nm light results predominantly in photodesorption, which limits the extent of possible surface photoreactions.

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