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 FeO
x
multilayer structure.
TPD shows that the NO binding energies vary significantly among the
Fe cation sites present on these FeO
x
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 ∼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 FeO
x
multilayer
islands within the FeO(111) monolayer, we observe a new NO TPD feature
at ∼200 K as well as diminution of the sharp TPD peak at 269
K. We speculate that these changes occur because the multilayer FeO
x
islands expose Fe3+ sites that
bind NO more weakly than the Fe2+ sites of the FeO monolayer.
RAIR spectra obtained from the NO-covered FeO
x
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
FeO
x
surfaces. This behavior suggests
that π-backbonding occurs to similar extents among the adsorbed
NO species, irrespective of the oxidation state and local structural
environment of the Fe surface site.