Adsorption of NO on FeOx Films Grown on Ag(111)

We used temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) to characterize the adsorption of NO on crystalline iron oxide films grown on Ag(111), including a Fe3O4(111) layer, an FeO(111) monolayer, and an intermediate FeOx multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeOx surfaces, and provides evidence that NO binds more strongly on Fe2+ sites than Fe3+ sites. The NO TPD spectra obtained from the Fe3O4(111) layer exhibit a dominant peak at 380 K, attributed to NO bound on Fe2+ sites, as well as a broad feature centered at similar to 250 K that is consistent with NO bound on Fe3+ sites of Fe3O4(111) as well as NO adsorbed on a minority FeO structure. The NO TPD spectra obtained from the monolayer FeO(111) film exhibits a prominent peak at 269 K. After growing FeOx multilayer islands within the FeO(111) monolayer, we observe a new NO TPD feature at similar to 200 K as well as diminution of the sharp TPD peak at 269 K. We speculate that these changes occur because the multilayer FeOx islands expose Fe3+ sites that bind NO more weakly than the Fe2+ sites of the FeO monolayer. RAIR spectra obtained from the NO-covered FeOx surfaces exhibit an N-O stretch band that blueshifts over a range from about 1800 to 1840 cm(-1) with increasing NO coverage. The measured N-O stretching frequency is only slightly red-shifted from the gas-phase value, and lies in a range that is consistent with atop, linearly bound NO on the Fe surface sites. In contrast to the NO binding energy, we find that the N-O stretch band is relatively insensitive to the NO binding site on the FeOx surfaces. This behavior suggests that pi-backbonding occurs to similar extents among the adsorbed NO species, irrespective of the oxidation state and local structural environment of the Fe surface site.