Adsorption of NO on FeO_<i>x</i> 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 FeO_<i>x</i> multilayer structure. TPD shows that the NO binding energies vary significantly among the Fe cation sites present on these FeO_<i>x</i> 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_<i>x</i> 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_<i>x</i> 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_<i>x</i> 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_<i>x</i> 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

The Role of Oxides in Catalytic CO Oxidation over Rhodium and Palladium

Catalytic CO oxidation is a seemingly simple reaction between CO and O₂ molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxide films are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures

Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal: Spatial Variation of Active and Poisoning Phases in Stationary Conditions

Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point, we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultrahigh vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off