Iron Single Crystal Surfaces Research Articles

Visualization of on-surface ethylene polymerization through ethylene insertion.

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level.

Evolution of Microstructure and Internal Stresses in Multi-Phase Oxide Scales Grown on (110) Surfaces of Iron Single Crystals at 650 °C

The evolution of microstructure and growth stresses in oxide scales grown on a (110) iron single crystal surface at 650 °C was studied by electron backscatter diffraction and in situ energy-dispersive diffraction with synchrotron radiation. Within this high temperature regime, the oxidation kinetics and scale microstructure were not significantly different from those encountered in the oxidation of ferrous polycrystals. Thus, epitaxial strains did not determine the stress state within the oxide scale. Relevant sources of growth stresses were inferred to be volumetric differences between the iron oxides in the early stages, and later, inner oxide formation, scale consumption as well as pore formation. These sources caused time-dependent stress cycles in magnetite and wüstite during oxidation. In the hematite layer stress cycles did not occur and creep appeared to be the predominant stress relieving mechanism. On cooling, the differences in thermal expansion caused residual stress gradients through the oxide scale.

The interaction of ammonia with small iron clusters: infrared spectra and density functional calculations of Fe n(NH 3) m and Fe n(ND 3) m complexes

Bare iron clusters react in the gas phase with ammonia to form Fe n (NH 3) m complexes. In the present study, the iron cluster–ammonia interaction within Fe n (NH 3) m and Fe n (ND 3) m complexes ( n=7–16) are investigated by molecular beam infrared depletion spectroscopy and density functional theory (DFT) calculations. Experimentally, we observe an absorption band within Fe n (ND 3) m complexes in the 880–890 cm −1 range, which is attributed to the ν 2 inversion mode of ND 3. DFT calculations performed for Fe n (NH 3) m and Fe n (ND 3) m model complexes ( n=1, 4, 7, and 13) predict that three of the four vibrational fundamentals of ammonia are only slightly shifted from their gas phase values, but that the symmetric ν 2 fundamental shifts substantially upward in the complex. For ND 3, ν 2 is predicted to shift from 748 cm −1 to the 850–900 cm −1 range when adsorbed to Fe n , as is observed experimentally in both the present cluster study and in iron single-crystal surface studies. DFT calculations of vertical ionization potentials of Fe n /Fe n (NH 3) m species and of Fe n –NH 3 binding energies are found to be in near-quantitative agreement with previously measured values.

Microcalorimetric measurements of differential heats of adsorption on reactive catalyst surfaces

Abstract Techniques are presented for measurement of differential heats of adsorption on reactive catalyst surfaces using heat-flux calorimetry. Samples are prepared ex-situ in ultra-pure flowing gases and then sealed in Pyrex capsules. Special calorimetric cells are employed to break the sample capsule after thermal equilibration of the sample with the calorimeter. In this manner the clean sample is exposed rapidly to the adsorbing gas, minimizing surface contamination. Initial heats of CO and H 2 adsorption at 403K on Pt SiO 2 catalysts obtained using the present technique (135 and 100 kJ/mol. respectively) were in agreement with results reported in the literature using standard calorimetric procedures. Initial heats measured in this study for CO adsorption at 308 K on reduced Ni powders (120 kJ/mol) and on nickel samples containing metallic potassium (200 kJ/mol) corresponded to values in the literature from ultrahigh vacuum studies of CO adsorption on Ni single crystal surfaces. The initial heat of N 2 adsorption at 453 K on reduced iron determined in this study (200 kJ/mol) was in agreement with results obtained in ultrahigh vacuum measurements of metallic iron single crystal surfaces. These results, for catalyst systems that are sensitive to traces of oxygen-containing species, provide strong evidence that the experimental techniques employed in the present study allow clean metallic surfaces to be maintained during microcalorimetric adsorption studies.

EECTRON TUNNELING PHENOMENA IN IRON OXIDES STUDIED WITH SCANNING TUNNELING MICROSCOPY AT 4. 2K

We have used STM to investigate the electron tunneling spectroscopy, as well as the tunneling barrier heights,in two kinds of iron oxides prepared differently on iron single crystal surfaces. The experimental results agree reasonably well with that of calculation employing stationary state tunneling method, and show that the iron oxide formed under ambient temperature (type Ⅰ) is mainly consisted of Fe3O4, while the surface processed by flash oxidation (type Ⅱ) should be consisted of high barrier oxides, most likely Fe2O3. Therefore, STM could be used to study various stages of surface oxidation on iron single crystal surfaces. The results aslo indicate that STM could be applied to study a range of metal oxides, and it is possible to investigate the internal structures of certain metal oxides possessing low barrier heights characteristics.

Effect of potassium on the high pressure kinetics of ammonia synthesis over fused iron catalyst

Measurements were performed of reaction rate in the process of ammonia synthesis (T=370–470°C) on doubly promoted (DP) (Al2O3, CaO) and triply promoted (TP) (K2O, Al2O3, CaO) iron catalysts. The latter were obtained by impregnation of the reduced and subsequently passivated DP precursors with alcoholic solution of KOH. The studies were carried out under high total pressure (10 MPa) in a wide range of ammonia partial pressure in the gas phase: from 0.25 to about 7 bar. The results are shown to be authoritative for the so-called kinetic regime. The effect of the presence of K+ cations in the catalyst was the stronger, as the temperature of the reaction was the lower and, in particular, the ammonia pressure in the gas phase the higher. The obtained results are in good accordance with the results of Somorjai's studies on activity of iron single crystal surfaces both clean and covered with (K+O) adlayer.

Potassium promotion of the ammonia synthesis reaction: A combined UHV-high pressure investigation

The ammonia synthesis rate has been measured as a function of the potassium coverage of a polycrystalline iron foil. The results clearly demonstrate a promoting effect of the potassium in agreement with the work of Ertl et al. on the dissociative dinitrogen adsorption at iron single crystal surfaces. However, the results also imply a marked difference between the state of the surface under ammonia synthesis conditions and during the dinitrogen adsorption experiments in ultra-high vacuum. It is suggested that in the former case the potassium atoms are surrounded by several strongly adsorbed atoms effectively blocking the corresponding sites for dinitrogen dissociation.

Ammonia-pretreatment-induced restructuring of iron single-crystal surfaces: Its effects on ammonia synthesis and on coadsorbed aluminum oxide and potassium

The effects of ammonia pretreatment on the structure and activity of iron single-crystal surfaces for ammonia synthesis are examined. Treatment of the (110), (100), and (111) planes of iron with 5 or 50 Torr of ammonia (723 K, 30 min) causes surface restructuring as evidenced by changes in ammonia synthesis activity (20 atm total pressure), and in temperature-programmed desorption, and by the appearance of new surfaces as seen by scanning electron microscopy. The (110) plane of iron is inactive for ammonia synthesis under the conditions used in this study, but the ammonia-pretreatment-restructured Fe(110) surface becomes about twice as active as the Fe(100) face. The restructured Fe(100) surface becomes about four times more active than the clean, unrestructured (100) surface. Restructuring the active Fe(111) face in ammonia results in a slight decrease in ammonia synthesis activity. The kinetic data, temperature-programmed desorption, and scanning electron microscopy suggest that Fe(110) and Fe(100) are restructured to surfaces containing C 7 sites, such as Fe(111) and Fe(211). A stereographic analysis of the restructured Fe(111) surface, using scanning electron microscopy, taken together with the reaction rate measurements suggests that {211} surface planes are formed upon ammonia treatment. The high concentrations of near-surface nitrogen that is deposited by the ammonia pretreatment does not block catalytic sites for the synthesis of ammonia. The presence of aluminum oxide on any of the iron single-crystal surfaces inhibits the ammonia-induced restructuring process, while the presence of potassium has no observable effect on the process. These results suggest a method of activating iron with both ammonia and water vapor restructuring in the presence of aluminum oxide and potassium to achieve a catalyst with optimal activity for ammonia synthesis.

Angularly resolved second harmonic generation study of the air formed oxide on iron

By variance of the angle of incidence and input polarization, it is possible to obtain structural information about the oxide layer that forms on iron surfaces from angularly resolved second harmonic generation (AR-SHG) experiments. The AR-SHG experiments on air-oxidized (110) iron single-crystal surfaces reveal the presence of both 2-fold and 3-fold symmetric species at the metal/oxide interface. These patterns are fit to an extension of existing theory, and the 3-fold symmetric oxide is found to be tilted by about 5{degree} from the Fe (110) plane. The 2-fold symmetric structure is aligned with the Fe (110) surface, and the SHG intensity contains both surface and bulk contributions.

On the rate enhancement of ammonia synthesis over iron single crystals by coadsorption of aluminum oxide with potassium

The behavior of doubly promoted iron catalysts utilized for ammonia synthesis is modelled by the coadsorption of aluminum oxide and potassium on iron single crystal surfaces that were employed in high pressure reaction rate studies. The promoter effect of aluminum oxide is due to its interaction with iron oxide during the preparation stage of the industrial catalyst. After reduction, aluminum oxide stabilizes the most active Fe(111) and Fe(211) crystal surfaces. Potassium does not appear to be involved in the structural promotion but its presence on the active iron surfaces increases the rate of dinitrogen dissociation mostly by lowering the concentration of adsorbed ammonia thus, making more catalytic sites available for dinitrogen dissociation. Co-adsorbed potassium and alumina form a potassium aluminate compound that a) inhibits the aluminum oxide induced restructuring of iron and b) covers up the active iron sites for ammonia synthesis.

The effects of potassium on ammonia synthesis over iron single-crystal surfaces

The effects of potassium on ammonia synthesis over model iron single-crystal catalysts of (111), (100), and (110) orientation have been studied under high-pressure reaction conditions (20 atm reactant pressure of nitrogen and hydrogen). Under these conditions, no more than 0.15 monolayers (ML) of potassium can be stabilized on the iron surfaces. The Fe(110) surface shows no activity for ammonia synthesis in this study with or without adsorbed potassium. The presence of potassium on the Fe(111) and Fe(100) surfaces increases the rate of ammonia synthesis markedly. At a low reaction conversion of 0.3% the rate over Fe(111) and Fe(100) is enhanced by a factor of two in the presence of potassium. The effect of potassium on Fe(111) and Fe(100) is enhanced as higher reaction conversions (i.e., increasing ammonia partial pressures) are achieved because potassium induces changes in the reaction orders for both ammonia and hydrogen. No change in the activation energy for the reaction is observed with potassium, suggesting that the reaction mechanism has not been altered. Temperature-programmed desorption shows that the adsorption energy of ammonia is significantly reduced when coadsorbed with potassium. A model is proposed in which the decrease of ammonia adsorption energy, induced by potassium, reduces the concentration of ammonia on the iron surface. This effect decreases the number of active sites blocked by the ammonia product, thereby increasing the rate of ammonia synthesis. The model also suggests that an additional effect of potassium is to increase the rate of nitrogen dissociative chemisorption by about 30% over Fe(111) and Fe(100) under ammonia synthesis conditions.

The effects of aluminum oxide in restructuring iron single crystal surfaces for ammonia synthesis

The effect of aluminum oxide and potassium on the ammonia synthesis over the (111), (100), and (110) faces of iron has been investigated. A restructuring of the Fe(110) and Fe(100) surfaces, induced by the presence of aluminum oxide and 20 Torr of water vapor, takes place making the restructured surfaces almost as active as the clean Fe(111) plane in the ammonia synthesis reaction (20 atm reactant pressure of hydrogen and nitrogen). The high activity of the restructured surfaces is maintained for over 4 hr of ammonia synthesis. Without the presence of aluminum oxide, treatment of the Fe(110) and Fe(100) surfaces with 20 Torr of water vapor again produces restructured surfaces which are almost as active as the Fe(111) plane for a short period. However, in this case deactivation of the restructured surfaces into the respective clean, unrestructured surfaces occurs within 1 hr of ammonia synthesis. Restructuring of the Fe(111) with 20 Torr of water vapor produces only a slight decrease in ammonia synthesis activity. The enhancement in rate of the restructured Fe(110) and Fe(100) surfaces, with or without aluminum oxide, might be explained by the formation of active surface orientations for ammonia synthesis (i.e., Fe(111) and Fe(211)), which contain C 7 sites (iron atoms with seven nearest neighbors), during the water vapor treatments. These restructured surfaces are only stable in the ammonia synthesis conditions when aluminum oxide is present. Potassium adsorbed alone or with coadsorbed aluminum oxide exhibits no promotional effects under the water vapor pretreatment conditions used in this study.

Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

Electron spectroscopic studies of tin surface segregation on iron single crystal surfaces

AES, ELS, LEED and XPS investigations of the surface segregation of tin dissolved in a Fe-4wt%Sn alloy were performed in ultra-high vacuum at elevated temperatures. The three low indexed surface orientations (100), (110) and (111) were studied. In all cases, no dependence of the maximum tin surface coverage on temperature was detected within the temperature range from 450 to 650°C. An order-disorder transition was observed by LEED, AES and XPS for the (100) oriented surface during tin segregation. The binding state for the segregated tin atoms abruptly changes at the order-disorder transition as determined by XPS. Similar results were obtained for the (111) surface. A deviating behaviour was observed for the (110) surface orientation, where two different ordered hexagonal surface structures were detected by LEED during tin surface enrichment. The first structure is similar to the diamond structure of pure tin, and the second one corresponds to the formation of a thin layer of the intermetallic compound FeSn on the (110) surface. The electron binding energies of the segregated tin atoms determined by XPS increase with increasing tin coverage on the (110) oriented surface. ELS studies on (100) and (111) oriented surfaces saturated with segregated tin show in comparison with literature data of pure tin a surface plasmon loss peak but no signal for the corresponding bulk loss. An energy loss signal found only for the (110) surface at Sn saturation coverage seems to be characteristic of an intermetallic FeSn surface phase.

Catalytic active sites on sputtered metal interfaces

By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied “π” as well as “σ” states at the Fermi level. Simple group theoretical arguments give that the “π” states, which represent the highest occupied molecular orbitals (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π ∗ , assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these “π” states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived “σ” states are introduced close to the Fermi level.

Catalytic active sites on sputtered metal interfaces

By the use of Hartree-Fock-Slater (HFS) cluster calculations, the symmetry of adatom induced electronic states and their possible role in catalytic processes have been investigated. For iron adsorbed on a close packed iron single crystal surface we find an increased density of occupied “π” as well as “σ” states at the Fermi level. Simple group theoretical arguments give that the “π” states, which represent the highest occupied molecular orbitais (HOMO) will mix with the lowest unoccupied molecular orbital (LUMO) of CO, 2π ∗, assuming CO bound normal to the surface in a terminal position with respect to the adatom. Such a system with an adsorbed atom on a flat surface is a model for a sputtered surface and these “π” states may thus explain the experimentally observed high rate of dissociation for CO on sputtered iron surfaces. A comparative study shows that no such increased density of states (DOS) is found when the Fe adatom is replaced by potassium. Finally for Cu adsorbed on Cu(111) only 4s derived “σ” states are introduced close to the Fermi level.

Adsorption of CO on Iron Single Crystal Surfaces

The adsorption of carbon monoxide on a stepped Fe(111) surface around 300 K was studied by low energy electron diffraction, thermal desorption spectroscopy and Auger electron spectroscopy. Similar detailed experiments on Fe(110) and (111), some of which were reported in Surf. Sci. 75 (1978) 46, were also performed. On a stepped (111) surface, CO adsorption produced a diffuse pattern very similar to the (1×1) pattern obtained with a flat (111) surface. Thermal desorption spectra from the stepped (111) surface showed two kinds of desorption states, α- and β-CO, as from the (110) and (111) surfaces, which are ascribed to molecularly and dissociatively adsorbed CO, respectively. The spectra also showed that the catalytic activity of C-O bond breaking was higher on the stepped (111) surface than on the (111) and (110) surfaces. The effect on CO adsorption of residual carbon on iron surfaces is almost negligible, while that of residual oxygen is high.

Adsorption of H2 on (110), (111) and Stepped (111) Iron Single Crystal Surfaces

The adsorption of hydrogen on Fe(110), (111) and stepped (111) surfaces has been studied using low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). Adsorption on the (110) surface around 300 K produces a variety of ordered LEED patterns. On the (111) surface, the LEED pattern remains unchanged with increasing exposure, whereas adsorption on the stepped (111) surface produces additional streaky spots in the direction normal to the steps. H2 desorption spectra from the (110) and (111) surfaces have a double β-peak, while the spectra from the stepped (111) surface, on the other hand, have only a single β-peak whose temperature decreases with increasing exposure. The initial heats of adsorption have been calculated at 24, 20.5 and 23 kcal/mole on the (110), (111) and stepped (111) surfaces, respectively. That the fractional order spots from the (110) surface covered with hydrogen are remarkably diffuse compared with those observed at lower temperatures may be explained in terms of the repulsive interaction between adatoms occupying the nearest sites. The interaction energy has been estimated at 1∼2 kcal/mole on this basis.

A permeaton cell for studying hydrogen desorption from iron single crystals into an UHV-system

A new hydrogen permeation cell is described which can be mounted on a normal UHV-manipulator. It is possible to change the pressure within the cell in the pressure range 10 -17 mbar to 1 bar with the aid of an additional external turbomolecular pump. The cell was constructed for studies of hydrogen desorption from iron single crystal surfaces. It is intended to investigate the effect of adsorbed and tarnishing layers on the hydrogen desorption, which tend to reduce the overall permeation coefficient of hydrogen in iron and steel [1, 2]. The surface layers are determined by AES on the UHV-side of the sample. For pure surfaces diffusion is controlling the hydrogen permeation. Then diffusion coefficients of hydrogen in iron single crystals can be evaluated from the measurements. The aim of our investigations is to study the hydrogen desorption from iron single crystal faces under controlled conditions at the exist surface. The influence of surface impurities such as oxygen, sulfur, carbon and nitrogen on the desorption process of hydrogen is to be determined at different temperatures.

Adsorption and segregation of sulphur, and its influence on the carburization and nitrogenation of iron and steel

The surface segregation of sulphur on iron was investigated by A.e.s. on iron polycrystals in the temperature range 700-1100 °C. Strong surface segregation occurs even at low bulk concentrations of 2—20 ug/g. The enthalpy of segregation for S(dissolved) = S (adsorbed) is about —165 kJ/mole S. By l.e.e.d. studies at 750 °C, ordered adsorption structures were observed on iron single crystal surfaces. In the segregation studies the equilibrium sulphur activity, a s , was determined by the concentration of dissolved sulphur. The results were correlated with the surface reaction kinetics in the carburization and nitrogenation of iron in the presence of adsorbed sulphur. These studies were performed in a flow apparatus at atmospheric pressure, the sulphur activity being established by the H 2 S/H 2 ratio in the gas atmosphere. The effect of adsorbed sulphur was studied for the reactions CH 4 (g) = C (dissolved) + 2H 2 (g) and N 2 (g) = 2N(dissolved) by a resistance relaxation method on thin iron foils at 800—900 °C. Even at bulk concentrations of about 0.1 ug/g S, the surface reactions were strongly retarded. Carbon and nitrogen transfer can only occur on reaction sites free of adsorbed sulphur. It can be seen from the kinetics that two sites are needed for the dissociation of N 2 but one site is sufficient for the decomposition of CH 4 . From the reaction kinetics, the adsorption isotherm for sulphur adsorption and segregation at 850 °C could be derived.