Surface Termination of Fe3O4(111) Films Studied by CO Adsorption Revisited

Interaction of water and carbon monoxide with MnO(001) thin films on Au(111)

Atomically defined MnO(001) thin films were grown on an Au(111) substrate, and their interaction with water (D2O) was investigated by infrared reflection absorption spectroscopy (IRAS) and thermal desorption spectroscopy (TDS). Carbon monoxide adsorption experiments were performed to probe surface atoms and defects on oxide films. Next, water interaction was investigated from which an associative binding pathway and a dissociative binding pathway were revealed, where the water molecules adsorb at terraces and water dissociation takes place at oxygen vacancies mediated by nearby Mn2+ sites. The IRAS data are supported by TDS experiments, which also manifest the importance of defects in the adsorption characteristics of MnO.

Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction

Water electrolysis to produce 'green H2' with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.

Oxygen-Terminated (1 × 1) Reconstruction of Reduced Magnetite Fe3O4(111)

The (111) facet of magnetite (Fe₃O₄) has been studied extensively by experimental and theoretical methods, but controversy remains regarding the structure of its low-energy surface terminations. Using density functional theory (DFT) computations, we demonstrate three reconstructions that are more favorable than the accepted Fe_oct2 termination under reducing conditions. All three structures change the coordination of iron in the kagome Fe_oct1 layer to be tetrahedral. With atomically resolved microscopy techniques, we show that the termination that coexists with the Fe_tet1 termination consists of tetrahedral iron capped by 3-fold coordinated oxygen atoms. This structure explains the inert nature of the reduced patches.

Defect engineering of oxide surfaces: dream or reality?

In this brief perspective we analyze the present status of the field of defect engineering of oxide surfaces. In particular we discuss the tools and techniques available to generate, identify, quantify, and characterize point defects at oxide surfaces and the main areas where these centers play a role in practical applications.

Surface structure of magnetite (111) under oxidizing and reducing conditions

We report on differences in the magnetite (111) surface structure when prepared under oxidizing and reducing conditions. Both preparations were done under UHV conditions at elevated temperatures, but in one case the sample was cooled down while keeping it in an oxygen atmosphere. Scanning tunneling microscopy after each of the preparations showed a different apparent morphology, which is discussed to be an electronic effect and which is reflected in the necessity of using opposite bias tunneling voltages in order to obtain good images. Surface x-ray diffraction revealed that both preparations lead to Fe vacancies, leading to local O-terminations, the relative fraction of which depending on the preparation. The preparation under reducing conditions lead to a larger fraction of Fe-termination. The geometric structure of the two different terminations was found to be identical for both treatments, even though the surface and near-surface regions exhibit small compositional differences; after the oxidizing treatment they are iron deficient. Further evidence for the dependence of iron vs oxygen fractional surface terminations on preparation conditions comes from Fourier transform infrared reflection-absorption spectroscopy, which is used to study the adsorption of formic acid. These molecules dissociate and adsorb in chelating and bidentate bridging geometries on the Fe-terminated areas and the signal of typical infrared absorption bands is stronger after the preparation under reducing conditions, which results in a higher fraction of Fe-termination. The adsorption of formic acid induced an atomic roughening of the magnetite (111) surface which we conclude from the quantitative analysis of the crystal truncation rod data. The roughening process is initiated by atomic hydrogen, which results from the dissociation of formic acid after its adsorption on the surface. Atomic hydrogen adsorbs at surface oxygen and after recombination with another H this surface hydroxyl can form H₂O, which may desorb from the surface, while iron ions diffuse into interstitial sites in the bulk.

DFT study of the fouling deposition process in the steam generator by simulating the adsorption of Fe2+ on Fe3O4 (0 0 1)

In order to reveal the fouling problem on the outer surface of the steam generator (SG) tube in the secondary circuit condition of pressurized water reactor (PWR) nuclear power plant, based on the density functional theory (DFT) method, the Cambridge sequential total energy program package (CASTEP) is used to simulate seven kinds of highly symmetric adsorption structure models of termination with tetrahedral Fe (A termination) and termination with octahedral Fe (B termination) on Fe₃O₄ (0 0 1) surface. The adsorption energies and stable adsorption conformations are calculated. The results show that the most stable adsorption structures of the Fe²⁺/Fe₃O₄ (0 0 1) configurations are Fe²⁺ above Fe-O bond of B layer termination (Fe₃O₄(001) A-b). During the adsorption, the Fe-Fe, Fe-O bond length, and Fe-Fe-O bond angle of (0 0 1) surface change, and the atomic positions parallel and perpendicular to (0 0 1) surface change correspondingly. The change happened to the surface layer is the most drastic one. The calculation of charge population, the density of states (DOS), and electron local function of Fe²⁺/Fe₃O₄ (0 0 1) optimal adsorption configuration show that there is electron transfer between Fe²⁺ and Fe₃O₄ (0 0 1), and the adsorption type is chemisorption. Among them, Fe (Fe²⁺)-Fe (Fe₃O₄) forms a metal bond, and Fe (Fe²⁺)-O (Fe₃O₄) forms the ionic bond. The results illustrate the interaction between free Fe²⁺ and Fe₃O₄ is the reason of the nucleation and agglomeration of Fe₃O₄ scale and it provides the foundation for the further research on Fe₃O₄ scale deposition.

Theoretical Study on the Lewis Acidity of the Pristine AlF3 and Cl-Doped α-AlF3 Surfaces

In the past two decades, metal fluorides have gained importance in the field of heterogenous catalysis of bond activation reaction, e.g., hydrofluorination. One of the most investigated metal fluorides is AlF3. Together with its chlorine-doped analogon aluminiumchlorofluoride (AlClxF3−x, x = 0.05–0.3; abbreviated ACF), it has attracted much attention due to its application in catalysis. Various surface models for α-AlF3 and their chlorinated analogues (as representatives of amorphous ACF) are investigated with respect to their Lewis acidity of the active centres. First-principle density functional theory (DFT) methods with dispersion correction are used to determine the adsorption structure and energy of the probe molecules CO and NH3. The corresponding vibrational frequency shift agrees well with the measured values. With this insight we predict the local structure of the active sites and can clarify the importance of secondary interactions to the local anionic surrounding of the catalytic site.

Heterogeneous Adsorption and Local Ordering of Formate on a Magnetite Surface

We report a novel heterogeneous adsorption mechanism of formic acid on the magnetite (111) surface. Our experimental results and density functional theory (DFT) calculations give evidence for dissociative adsorption of formic acid in quasibidentate and chelating geometries. The latter is induced by the presence of iron vacancies at the surface, making oxygen atoms accessible for hydrogen atoms from dissociated formic acid. DFT calculations predict that both adsorption geometries are energetically favorable under our experimental conditions. The calculations prove that the locally observed (√3 × √3)R 30° superstructure consists of three formate molecules in a triangular arrangement, adsorbed predominantly in a chelating geometry. The results show how defects can stabilize alternative adsorption geometries, which is a crucial ingredient for a detailed atomistic understanding of reaction barriers on magnetite and other oxide surfaces, as well as for the stability of carboxylic acid based nanocomposite materials.

Machine Learning in Computational Surface Science and Catalysis: Case Studies on Water and Metal-Oxide Interfaces

The goal of many computational physicists and chemists is the ability to bridge the gap between atomistic length scales of about a few multiples of an Ångström (Å), i. e., 10⁻¹⁰ m, and meso- or macroscopic length scales by virtue of simulations. The same applies to timescales. Machine learning techniques appear to bring this goal into reach. This work applies the recently published on-the-fly machine-learned force field techniques using a variant of the Gaussian approximation potentials combined with Bayesian regression and molecular dynamics as efficiently implemented in the Vienna ab initio simulation package, VASP. The generation of these force fields follows active-learning schemes. We apply these force fields to simple oxides such as MgO and more complex reducible oxides such as iron oxide, examine their generalizability, and further increase complexity by studying water adsorption on these metal oxide surfaces. We successfully examined surface properties of pristine and reconstructed MgO and Fe₃O₄ surfaces. However, the accurate description of water–oxide interfaces by machine-learned force fields, especially for iron oxides, remains a field offering plenty of research opportunities.

Gold nanostructures on iron oxide surfaces and their interaction with CO

We review results of density functional theory calculations of the adsorption of single gold atoms and formation of sub-nanometer Aunstructures (n= 2 to 5) on most stable iron oxide surfaces: hematite (0001), and magnetite (111) and (001). Structural, energetic, and electronic properties of Aunstructures on both Fe- and O-rich oxide terminations are discussed. Different chemical character of the two oxide terminations is reflected in distinctly stronger binding of gold at the oxygen- than at the iron-terminated surface, and in different changes of the adsorption binding energy with the size of the Auncluster. On the iron-terminated oxide surface the binding energy increases whereas on the oxygen-rich termination it decreases with the number of Au atoms in the structure. Upon CO adsorption on magnetite surface all Aunstructures have a net positive charge and CO binds to the most cationic Au atom of a cluster. Interactions of Aunand CO with magnetite (111) show many similarities with those on hematite (0001) surface. The influence of the substrate relaxation effects on adsorption energy is also discussed.

Vibrational properties of CO2 adsorbed on the Fe3O4 (111) surface: Insights gained from DFT

By virtue of density functional theory calculations, this work discusses several carbonate, carboxylate, and bicarbonate species on two thermodynamically relevant metal terminations of the (111) surface of magnetite, Fe₃O₄. We present adsorption energies and vibrational wavenumbers and conclude in assigning the observed infrared reflection-absorption spectroscopy bands. CO₂ prefers to adsorb molecularly on the Fe_tet1 terminated Fe₃O₄(111) surface, a finding consistent with observation. Calculations compared with the experiment lead to interpreting results in favor of the Fe_tet1 (single metal) terminated Fe₃O₄(111) surface as the regular surface termination. Formation of carbonate and bicarbonate requires metal impurities on that surface. Such impurities exist, for instance, on the Fe_oct2 (double metal) termination, which can thus be used as a model for "metal-rich terminations" of more complex surfaces.

Elucidating Surface Structure with Action Spectroscopy

Surface Action Spectroscopy, a vibrational spectroscopy method developed in recent years at the Fritz Haber Institute is employed for structure determination of clean and H₂O-dosed (111) magnetite surfaces. Surface structural information is revealed by using the microscopic surface vibrations as a fingerprint of the surface structure. Such vibrations involve just the topmost atomic layers, and therefore the structural information is truly surface related. Our results strongly support the view that regular Fe₃O₄(111)/Pt(111) is terminated by the so-called Fe_tet1 termination, that the biphase termination of Fe₃O₄(111)/Pt(111) consists of FeO and Fe₃O₄(111) terminated areas, and we show that the method can differentiate between different water structures in H₂O-derived adsorbate layers on Fe₃O₄(111)/Pt(111). With this, we conclude that the method is a capable new member in the set of techniques providing crucial information to elucidate surface structures. The method does not rely on translational symmetry and can therefore also be applied to systems which are not well ordered. Even an application to rough surfaces is possible.

Dual Lewis site creation for activation of methanol on Fe3O4(111) thin films

Despite a wide application in heterogeneous catalysis, the surface termination of Fe₃O₄(111) remains controversial. Herein, a surface with both Lewis acid and base sites is created through formation of an Fe₃O₄(111) film on α-Fe₂O₃(0001). The dual functionality is generated from a locally nonuniform surface layer of O adatoms and Fe_tet1 sites. This reactive layer is reproducibly formed even in oxygen-free environments because of the high mobility of ions in the underlying α-Fe₂O₃(0001). The atomic structure of the Fe₃O₄(111) surface was identified by scanning tunneling microscopy (STM) and density functional theory (DFT) using the registry of the overlayers with the surface and the distinct electronic structure of oxygen adatom (O_ad) and uncovered lattice Fe_tet1. The surface is dominated by the interface of O_ad and Fe_tet1, a Lewis acid–base pair, which favors methanol dissociation at room temperature to form methoxy. Methoxy is further oxidized to yield formaldehyde at 700 K in temperature programmed reaction spectra, corresponding to an approximate activation barrier of 179 kJ mol⁻¹. The surface termination of Fe₃O₄(111) is fully recovered by rapid heating to 720 K in vacuum, demonstrating the high mobility of ions in this material. The work establishes a clear fundamental understanding of a unique magnetite surface and provides insights into the origin of selective oxidation of alcohols on magnetite-terminated catalysts.

Adjacent oxygen adatoms and lattice iron atoms on magnetite surfaces serve as dual Lewis sites that oxidize methanol to formaldehyde.

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe₂O₃(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar⁺-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe₃O₄(111)/Fe_1−xO(111)-biphase exposing both, Fe³⁺ and Fe²⁺ surface species. Here the well-defined surface domains of Fe₃O₄(111) exhibit a Fe_oct2-termination, while the reduced Fe_1−xO(111) is Fe²⁺(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

A Computational Study of AlF3 and ACF Surfaces

By applying first principles density functional theory (DFT) methods, different metal fluorides and their surfaces have been characterized. One of the most investigated metal fluorides is AlF3 in different polymorphs. Its chloride-doped analogon AlClxF3−x (ACF) has recently attracted much attention due to its application in catalysis. After presenting a summary of different first-principle studies on the bulk and surface properties of different main group fluorides, we will revisit the problem of the stability of different α -AlF3 surfaces and extend the investigation to chloride-doped counterparts to simulate the surface properties of amorphous ACF. For each material, we have considered ten different surface cuts with their respective terminations. We found that terminations of ( 01 1 ¯ 0 ) and ( 11 2 ¯ 0 ) yield the most stable surfaces for α -AlF3 and for the chlorine substituted surfaces. A potential equilibrium shape of the crystal for both α -AlF3 and ACF is visualized by a Wulff construction.

Water adsorption on the Fe3O4(111) surface: dissociation and network formation

Water adsorption on Fe₃O₄(111) is studied in detail using infrared spectroscopy, temperature programmed desorption, micro-calorimetry and density functional theory.