FeO Islands Research Articles

The selective blocking of potentially catalytically active sites on surface-supported iron oxide catalysts

Selective growth of Pd and Pt on the edges of Au(111) supported FeO islands was observed. The complicated interfaces formed in selective growth raise a platform for the research of edge involved catalytic reactions.

Reversible oxidation and reduction of gold-supported iron oxide islands at room temperature

Monolayer iron oxides grown on metal substrates have widely been used as model systems in heterogeneous catalysis. By means of ambient-pressure scanning tunneling microscopy (AP-STM), we studied the in situ oxidation and reduction of FeO(111) grown on Au(111) by oxygen (O2) and carbon monoxide (CO), respectively. Oxygen dislocation lines present on FeO islands are highly active for O2 dissociation. X-ray photoelectron spectroscopy measurements distinctly reveal the reversible oxidation and reduction of FeO islands after sequential exposure to O2 and CO. Our AP-STM results show that excess O atoms can be further incorporated on dislocation lines and react with CO, whereas the CO is not strong enough to reduce the FeO supported on Au(111) that is essential to retain the activity of oxygen dislocation lines.

Vibrational fingerprint of the catalytically-active FeO2-x iron oxide phase on Pt(1 1 1)

We report here on the oxidation of monolayer FeO islands on Pt(1 1 1) into the highly-reactive FeO2-x phase by high-temperature oxidation under Ultra High Vacuum (UHV) conditions. The chemical composition and characteristic phonon modes of both FeO and FeO2-x films were investigated by X-ray photoemission Spectroscopy (XPS) and High Resolution Electron Energy Loss Spectroscopy (HREELS). The reactivity and thermal stability of the “oxygen-rich” FeO2-x phase were studied with respect to the CO oxidation reaction at 450 K and to annealing in UHV at 563 K, respectively. By performing repeated oxidation/reduction cycles, we have identified the vibrational mode at 76 meV as the unique signature of the FeO2-x phase and confirmed the already reported ability of CO to reduce such oxide at 450 K. The latter process is not fully reversible, suggesting a modification of the active sites upon CO adsorption.

Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands.

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O2) and hydrogen (H2), respectively. The FeO islands respond very dynamically toward O2, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O2 dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O2 molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H2 molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H2 dissociation. Our STM results reveal how excess O atoms are incorporated and released in O2 and H2 and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.

Active Phase of FeOx/Pt Catalysts in Low-Temperature CO Oxidation and Preferential Oxidation of CO Reaction

The interface between metal and reducible oxide has attracted increasing interest in catalysis. The FeOx–Pt interface has been a typical example, which showed remarkable activity for the preferential oxidation of CO (PROX) at low temperatures. However, model catalytic studies under vacuum conditions or in high-pressure O-rich environment at 450 K have reported two different active phases with iron in two different valence states, invoking a possible pressure gap. To identify the active phase for low-temperature CO oxidation and PROX, it is necessary to investigate the stability and activity of FeO/Pt(111) under the realistic reaction conditions. We thus conducted an in situ study on FeO/Pt(111) from ultrahigh vacuum to the atmospheric pressure of reactant gases. Our study shows FeO islands were easily oxidized in 1 Torr O2 to form the trilayer FeO2 islands. However, the presence of 2 Torr CO could prevent the oxidation of FeO islands and lead to CO oxidation at the FeO/Pt(111) interface. The FeO/Pt(111) s...

Identification of O-rich structures on platinum(111)-supported ultrathin iron oxide films

Using high-resolution scanning tunneling microscopy (STM) we have studied the oxidation of ultrathin FeO films grown on Pt(111). At the initial stage of the FeO film oxidation by atomic oxygen exposure, we identified three distinct types of line defects, all of which form boundaries between FeO domains of opposite orientation. Two types of line defects appearing bright (type-i) and dark (type-ii) in the STM images at typical scanning parameters are “metallic”, whereas the third line defect exhibits nonmetallic behavior (type-iii). Atomic-scale structure models of these line defects are proposed, with type-i defects exhibiting 4-fold coordinated Fe atoms, type-ii exhibiting 2-fold coordinated O atoms, and type-iii exhibiting tetrahedrally-coordinated Fe atoms. In addition, FeO2 trilayer islands are formed upon oxidation, which appear at FCC-type domains of the moiré structure. At high scanning bias, distinct protrusions on the trilayer islands are observed over surface O ions, which are assigned to H adatoms. The experimental data are supported by density functional theory (DFT) calculations, in which bare and hydroxylated FeO2 trilayer islands are compared. Finally, we compare the formation of O-rich features on continuous FeO films using atomic oxygen with the oxidation of Pt(111)-supported FeO islands accomplished by O2 exposure.

Atomic Layer Deposition of FeO on Pt(111) by Ferrocene Adsorption and Oxidation

We report the synthesis of a submonolayer film of FeO(111) on the Pt(111) surface using atomic layer deposition under well-defined ultrahigh vacuum (UHV) conditions. FeO islands were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy. FeO(111) on Pt(111) was prepared through adsorption and oxidation of ferrocene. Because ferrocene adsorption is crucial to the overall process, it was studied in detail at different exposures and temperatures. At low exposures and at 300 K, ferrocene was found to adsorb dissociatively mainly as Cp (cyclopentadienyl ring). At high exposures, molecular adsorption dominated, and the dissociation fragments were replaced with ferrocene molecules. The adsorbed ferrocene desorbed from the Pt(111) surface at a temperature between 473 and 573 K. FeO(111) islands were grown by exposing the ferrocene adlayer to 1 × 10–6 mbar O2 at 623 K resulted in islands with the shape of truncated triangles an...

Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface.

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations. Time-lapsed STM imaging on this inverse model catalyst in O2 and CO environments revealed catalytic activity occurring at the FeO-Pt(111) interface and directly showed that the Fe-edges host the catalytically most active sites for the CO oxidation reaction. This is an important result since previous evidence for the catalytic activity of the FeO-Pt(111) interface is essentially based on averaging techniques in conjunction with DFT calculations. The presented STM results are in accord with DFT+U calculations, in which we compare possible CO oxidation pathways on oxidized Fe-edges and O-edges. We found that the CO oxidation reaction is more favorable on the oxidized Fe-edges, both thermodynamically and kinetically.

Unraveling the edge structures of platinum(111)-supported ultrathin FeO islands: the influence of oxidation state.

We used high-resolution scanning tunneling microscopy to study the structure of ultrathin FeO islands grown on Pt(111). Our focus is particularly on the edges of the FeO islands that are important in heterogeneous catalysis, as they host the active sites on inversed catalysts. To imitate various reaction environments we studied pristine, oxidized, and reduced FeO islands. Oxidation of the FeO islands by O2 exposure led to the formation of two types of O adatom dislocations and to a restructuring of the FeO islands, creating long O-rich edges and few short Fe-terminated edges. In contrast, reducing the FeO islands led to a dominance of Fe-rich edges and the occurrence of few and short O-rich edges. In addition, for reducing conditions we observed the formation of O vacancy dislocations on the FeO islands. Through the identification of O adatom and O vacancy dislocations known from closed ultrathin FeO films and geometrical considerations we unraveled the atomic structure of the predominant FeO boundaries of pristine, oxidized, and reduced FeO islands. The results indicate an astonishing flexibility of the FeO islands on Pt(111), since the predominant edge termination and the island shape depend strongly on the preparation conditions.

Initial stages of FeO growth on Ru(0001)

We study how FeO wüstite films on Ru(0001) grow by oxygen-assisted molecular beam epitaxy at elevated temperatures (800–900 K). The nucleation and growth of FeO islands are observed in real time by low-energy electron microscopy (LEEM). When the growth is performed in an oxygen pressure of 10−6 Torr, the islands are of bilayer thickness (Fe–O–Fe–O). In contrast, under a pressure of 10−8 Torr, the islands are a single FeO layer thick. We propose that the film thickness is controlled by the concentration of oxygen adsorbed on the Ru. More specifically, when monolayer growth increases the adsorbed oxygen concentration above a limiting value, its growth is suppressed. Increasing the temperature at a fixed oxygen pressure decreases the density of FeO islands. However, the nucleation density is not a monotonic function of oxygen pressure.

CO Oxidation at the Perimeters of an FeO/Pt(111) Interface and how Water Promotes the Activity: A First‐Principles Study

The catalytic role of the Pt--Fe cation ensemble presented at the perimeters of the FeO film supported on Pt(111) for low-temperature CO oxidation and the promotion of water on activity were studied by using DFT calculations. We found that the perimeter sites along the edge of the FeO islands on Pt provided a favorable ensemble that consisted of coordinatively unsaturated ferrous species and nearby Pt atoms for O(2) and H(2) O activation free from CO poison. A dissociative oxygen atom at the Pt--Fe cation ensemble reacts easily with CO adsorbed on nearby Pt. The OH group from water dissociation not only facilitates activation of the oxygen molecule, more importantly it opens a facile reaction channel for CO oxidation through the formation of the carboxyl intermediate. The presence of the OH group on the FeO film strengthens interfacial interactions between FeO and Pt(111), which would make the FeO film more resistant to further oxidation. The importance of the Pt--Fe cation ensemble and the role of water as a cocatalyst for low-temperature CO oxidation is highlighted.

Observation of water dissociation on nanometer-sized FeO islands grown on Pt(1 1 1)

Adsorption and dissociation of water on nanometer-sized FeO islands on Pt(1 1 1) substrate were investigated using scanning tunneling microscopy. It is observed that the adsorbed water molecules may spontaneously dissociate into hydroxyls on the step edges of the FeO(1 1 1) islands, even at 80 K, which can be attributed to the active sites of coordinatively unsaturated Fe atoms at the step edges. These hydroxyls are found to diffuse and form chains on the FeO terrace. The formation of the oxygen vacancy chains on FeO islands indicates that the hydroxyls may desorb from the surface as H 2 and H 2O at room temperature.

Formation of Periodic Arrays of O Vacancy Clusters on Monolayer FeO Islands Grown on Pt(111)

The structural evolution of a Pt surface with 0.4 monolayer (ML) subsurface Fe on annealing in 1.1 × 10 −7 kPa O 2 was studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. When the annealing temperature was 600 K, only dissociative adsorption of O 2 occurred, which induced the local restructuring of surface regions. At 750 K, subsurface Fe atoms segregated onto the surface and were oxidized. When the annealing temperature was increased to 850 K, well defined monolayer FeO islands with periodic arrays of defects were formed. The defects were located at the hexagonal closest packed sites (fcc) of FeO Moiré unit cells and were either single oxygen vacancies or multiple oxygen vacancies consisting of six missing O atoms. The formation of periodic defects on monolayer FeO islands may be a way to construct active sites on Pt-Fe model catalysts.

High temperature oxidation of thin CrN coatings deposited on steel

Chromium nitride coatings were deposited on steel substrates by the tetrode plasma-beam sputtering technique at 200 °C. The coatings exhibit a dense, smooth, and homogeneous structure with the appearance of occasional pinhole defects, as shown by scanning electron microscopy (SEM) and atomic force microscopy. The thickness of the coatings was sufficiently small (0.34 μm) to allow the study of the compositional changes induced by the high temperature oxidation throughout the oxide and nitride phases, as well as at the inner interface between the nitride coating and the substrate. The CrN coatings were oxidized in an oxygen flow at temperatures up to 750 °C. After oxidation at temperatures higher than 450 °C the formation of a Cr2O3 layer as a separate phase on the top of the nitride coating is observed. It is suggested that the mechanism of CrN oxidation proceeds by the progressive replacement of nitrogen by oxygen. As the temperature is raised the thickness of the Cr2O3 layer increases and its grain structure becomes more pronounced. Besides oxidation of CrN to Cr2O3 another parallel process is taking place, namely the oxidation of iron from the substrate. This process leads to the formation of FeO islands, which represent defect spots on the surrounding Cr2O3 surface. The number of these defects depends on the temperature and time of oxidation. In order to study the failure mechanism, various surface analytical techniques were applied, i.e., x-ray photoelectron-spectroscopy, Auger electron spectroscopy, energy dispersive x-ray spectroscopy, and SEM. It is suggested that the penetration of iron through the coating starts at the pinholes present in the coating. At higher temperatures, however, this is not the dominating mechanism, and the diffusion process is further facilitated by the mismatch in the thermal expansion coefficients between the substrate and the nitride coating. Thermal stress thus established leads to the loss of adhesion at the inner substrate/coating interface and therefore creates additional paths for the diffusion of iron.

Initial stages of Fe(110) oxidation at 300 K: kinetics and structure

STM has been used to study the structural changes occurring during O2 chemisorption on Fe(110) and to correlate these with sticking and LEED measurements [A. Hodgson et al., Surf. Sci. 319 (1994) 119]. Oxygen forms and ordered c(2 × 2) phase with a coverage of 0.25 monolayer which at higher coverages is replaced by a disordered layer with local features having a (3 × 1) periodicity, aligned to the close packed directions. At O coverages above ∼ 0.25 monolayer the surface shows small single step height features on the O-rich areas of the surface, attributed to FeO. As the O coverage increases above 0.4 monolayer the size and density of the FeO islands increases, finally giving a hexagonal LEED pattern characteristic of FeO(111). The onset of FeO nucleation coincides with a minimum in the O2 sticking probability.