Roles of Fe2+, Fe3+, and Cr3+ surface sites in the oxidation of NO on the (Fe,Cr)3O4(111) surface termination of an α-(Fe,Cr)2O3(0001) mixed oxide

The oxidation of NO was explored on a mixed Fe + Cr oxide surface using temperature-programmed desorption (TPD). NO desorbs from (Fe,Cr)(3)O-4(111) in two main peaks at 220 and 370 K, with a third minor peak at similar to 315 K. O-2 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 similar to 10% of the adsorbed O-2 dissociated at Fe2+ sites. NO reacts with preadsorbed O-2 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 + O-2 with 460 nm light results predominantly in photodesorption, which limits the extent of possible surface photoreactions. (C) 2014 Elsevier Inc. All rights reserved.