Clean Fe Surface Research Articles

Computational demystification of iron carbonyls formation under syngas environment

Iron pentacarbonyl (IPC) gas forms upon the reaction of carbon monoxide with Fe containing metallic surfaces under gas reforming conditions. IPC formation can sometimes reach alarming levels that cause metal loss, pipeline thinning corrosion, catalyst poisoning, and contamination of sensitive industrial equipment. In this work, we demystify using multiscale computational modeling the mechanism of Iron pentacarbonyl formation: Density functional theory (DFT) is used to explore various catalytic reactions that involve a Fe adatom reacting with adsorbed carbon monoxide. Our calculated carbonyls desorption barriers on a perfect and clean Fe surface are too high to allow the carbonyls to form then desorb at temperatures <500 K at the rates reported experimentally. Most importantly, our calculations indicate that a high CO surface coverage, in addition to the presence of Fe adatoms, favors carbonyl formation and its desorption towards the flowing gas medium. Using insights extracted from ab initio molecular dynamics simulations, we propose that the most plausible IPC formation mechanism consists of: (1) on surface reactions of adsorbed CO molecules with an Fe adatom to form iron tricarbonyl (Fe(CO)3*) molecules; (2) an adsorbate assisted movement of iron tricarbonyl on top of the CO adlayer; and (3) the interaction of iron tricarbonyl with CO molecules from the gaseous medium eventually leading to iron adatom removal as Fe(CO)5 gas.

Insights on the effect of water content in carburizing gas mixtures on the metal dusting corrosion of iron

Constituents of syngas, such as water, carbon monoxide and sulfides, can cause the degradation of the steel pipes they move through, leading to carbon dusting and corrosion. In spite of considerable attention to this process, many questions remain about its origin. We conduct reactive molecular dynamics simulations of multi-grain iron systems exposed to carburizing gas mixtures to investigate the effect of water content on metal dusting corrosion. To simulate carbon monoxide (CO) dissociation followed by carbon diffusion, we employ an extended-ReaxFF potential that allows accounting for both the high C atoms coordination in bulk iron as well as the lower C coordination at the iron surface and interfaces. The reactions happening in the sample at different water concentrations and at different time frames are explored. We demonstrate that the presence of water on a clean Fe surface promotes different catalytic reactions at the beginning of the simulations that boost the C, H, O diffusion into the sample. At later stage, the formation of oxide scale leads to an elevated concentration of H2O/OH molecules on the surface due to the decrease in Fe affinity to dissociate water. This results into blocking the Fe catalytic sites leading to lower C and O diffusion to the bulk of the sample.

Chemical Degradation of a Multiply Alkylated Cyclopentane (MAC) Oil During Wear: Implications for Spacecraft Attitude Control System Bearings

The wear-induced chemical degradation process was examined using deliberately under-lubricated thrust ball bearings to create wear in a reasonable time frame. The liquid-phase products were analyzed using infrared spectroscopy and gel-permeation chromatography. Our results indicate that oil molecules react with the freshly created clean Fe surface produced during wear, and fragment into radicals by breaking C–C bonds. These free radicals produce alkenes, which can then be further attacked by radicals, leading to the formation of larger molecules (oligomers and eventually polymers). A side reaction, production of iron carboxylate salts, also occurred during wear. Additional experiments indicate that organic acids can be produced by radical reaction with the oxidized areas of the bearing surfaces and wear particles, and these acids can then attack the clean Fe surfaces created by wear to form iron carboxylate salts. We used our experimental results to estimate the amount of degradation likely to be produced in attitude control system bearing tests under conditions that produce wear and poor performance. We find that multiply alkylated cyclopentane (MAC) oil chemical degradation is an effect of wear, and is insufficient to be a cause of failure itself. Life predictions for MAC-lubricated attitude control system bearings must be based on predictions of oil transport and wear since failures are due to the failure of the lubrication system, not of the lubricant itself.

Observation of a high negative spin polarization at the Fe/MgO(100) surface oxidized at room temperature

Spin polarization of an Fe/MgO(100) surface oxidized at room temperature has been analyzed with a spin-polarized metastable helium beam, which is an extremely surface sensitive spin probe, under high magnetic fields (0–50 kOe). The spin polarization of the Fe surface, although it initially decays with oxygen exposure, increases at &amp;gt;20 L (1 L=10−6 Torr sec) and becomes comparable to that observed for a clean Fe surface at &amp;gt;100 L. The polarization is negative and especially high at around the Fermi level. This may be understood if we assume the formation of Fe3O4.

Magnetization measurement of Pd(1 0 0) using a spin-polarized metastable He beam and an electromagnet of 1.8 kOe

Using a spin-polarized metastable helium ( He * ) beam, which is an extremely surface-sensitive spin probe, we made an attempt to detect the electron spin polarization for a clean Pd(1 0 0) surface which is often assumed to be ferromagnetic in nano-particle studies. The spin asymmetry in the total electron yield measured at 90 K with an external magnetic field of 1.5 kOe was however, less than 1 100 of that for a clean Fe surface.

The surface chemistry of vapor phase lubricants: tricresylphosphate on Ni(1 0 0)

Tricresylphosphate (TCP) is known to serve as an excellent high temperature vapor phase lubricant with some metals such as Fe but not with others such as Ni. The surface chemistry of m-TCP has been studied on clean and phosphorous covered Ni(1 0 0) surfaces in order to understand the differences between its reactivity on Fe and Ni. Our results show that upon heating to 800 K m-TCP decomposes on the clean Ni(1 0 0) surface to deposit carbon and phosphorous with the evolution of H 2, CO, benzene, and toluene into the gas phase. During further heating to 1000 K, all the carbon on the surface dissolves into the Ni bulk leaving only phosphorous. The adsorption and heating of m-TCP on the phosphorous-modified Ni surface does not result in significant decomposition. The clean Ni substrate is able to activate TCP decomposition in much the same way that it decomposes on the clean Fe surface. The lack of significant differences in the chemistry of TCP on the clean metal surfaces cannot explain the fact that TCP vapor will form thick lubricating films on Fe but not on Ni.

Growth of face-centered-cubic Fe films on Cu(100): Interdiffusion, surfactant effects, and island formation

Various factors complicating the growth of ultrathin films of \ensuremath{\gamma}-Fe on Cu(100)---interdiffusion, formation of surfactant layers, and island formation---have been investigated using density-functional calculations. It is shown that the formation of a surfactant Cu layer on top of the growing Fe films is energetically favored over both clean Fe surfaces and the formation of ordered surface alloys. However, the exchange process leading to the incorporation of the Fe atoms in or below the Cu top layer is characterized by a barrier of 1.45 eV/Fe atom, i.e., it will be kinetically hindered at low temperatures. In the initial stages of Fe deposition, as long as parts of the surface of the substrate remain uncovered by Fe, island formation is energetically preferred over the formation of atomically flat films. A surfactant Cu layer largely suppresses this tendency to surface roughening. Surface energies of 0.60 eV/atom for Cu(100) and of 0.84 eV/atom for \ensuremath{\gamma}-Fe(100) have been calculated and the difference is identified as the driving mechanism behind these phenomena.

H 2S adsorption onto clean and oxygen covered iron films

Auger electron spectroscopy (AES) and the chemisorption-induced resistance change (CIRC) have been employed to compare the kinetics of low pressure H 2S adsorption on clean and oxygen-covered Fe films. Although the oxygen overlayer does not appear to affect the saturation coverage of adsorbed H 2S, it does significantly enhance the adsorption rate when compared with the clean Fe surface. Differences are also observed in the sulfur Auger spectrum obtained from the two surfaces. It is suggested that the differences observed in the adsorption rates and the shapes of the uptake curves result from a change in the rate-limiting adsorption step; from island growth for the clean Fe surface to molecular-precursor mediated adsorption on the oxygen-covered Fe surface. The adsorption of H 2S did not lead to any obvious corrosion of the oxygen overlayer at the temperature of these measurements, about 295 K.

Two-dimensional spin-polarized states of Ag on Fe(100).

We present an experimental and theoretical investigation of the spin-dependent electronic structure of a Ag monolayer on Fe(100). By spin- and angle-resolved photoemission we identify several spin-polarized states of two-dimensional (2D) character and determine their band dispersion. The experimental results are compared with ab initio band-structure calculations. Three kinds of spin-polarized 2D states could be identified along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{}-X\ifmmode\bar\else\textasciimacron\fi{} high-symmetry direction of the surface Brillouin zone: (i) 3d-derived Fe surface states which are only weakly modified by the Ag overlayer; (ii) Fe surface states with a substantial sp character which, upon adsorption of a Ag overlayer, hybridize with the Ag atoms and form interface states; (iii) Ag-induced spin-polarized interface states which are shared by both of the constituents of the interface atoms and do not have any counterpart at the clean Fe surface.

Layer-dependent magnetic properties at Fe surfaces using spin-polarized electron emission spectroscopy (SPEES)

We have studied the angle-resolved energy distribution of spin-polarized electrons emitted during ion-surface scattering. At clean Fe surfaces, electron spin polarization values (ESP) of up to 48% are found. Varying the scattering angle of the ions from 1° to 45°, we find that the layer-dependent, average ESP at the topmost surface layer is enhanced by 32% compared to that of the bulk and that the mean free path of electrons with energy 4–10 eV is about 3 atomic distances. The absence of a magnetically dead layer at Fe surfaces covered with one monolayer of O is revealed by the existence of a nonzero surface ESP.

Hydroxyl stabilization on iron surfaces by chlorine interactions

We have used X-ray photoelectron spectroscopy (XPS) to study the surface chemistry of H 2O and chlorine on a polycrystalline Fe surface. Our results indicate that the sequence in which the sample is exposed to H 2O and chlorine significantly alters the interaction on the Fe surface. On Cl 2/H 2O/Fe (i.e., exposed to H 2O first) disproportionation of Cl 2 is observed resulting in formation of HCl and a OHCl complex, (OHCl) a. HCl desorbs from the surface adlayer below 200 K. An important result is that (OHCl) a is stable on the surface to about 700 K in sharp contrast to OH adsorbed on a clean Fe surface, where desorption occurs below 300 K under UHV conditions. On the H 2O/Cl 2/Fe surface OH also desorbs below 300 K, indicating no ClOH stabilization. The effect of dosing sequence is attributed to the disproportionation of Cl 2 within the Cl 2/H 2O/Fe adlayer. Initial adsorption of Cl 2 results instead in FeCl formation and weak interaction with adsorbed OH. The results emphasize the importance of solvation effects on impurity induced H 2O-surface interactions.

Hydroxyl stabilization on iron surfaces by chlorine interactions

We have used X-ray photoelectron spectroscopy (XPS) to study the surface chemistry of H 2O and chlorine on a polycrystalline Fe surface. Our results indicate that the sequence in which the sample is exposed to H 2O and chlorine significantly alters the interaction on the Fe surface. On Cl 2/H 2O/Fe (i.e., exposed to H 2O first) disproportionation of Cl 2 is observed resulting in formation of HCl and a OHCl complex, (OHCl) a. HCl desorbs from the surface adlayer below 200 K. An important result is that (OHCl) a is stable on the surface to about 700 K in sharp contrast to OH adsorbed on a clean Fe surface, where desorption occurs below 300 K under UHV conditions. On the H 2O/Cl 2/Fe surface OH also desorbs below 300 K, indicating no ClOH stabilization. The effect of dosing sequence is attributed to the disproportionation of Cl 2 within the Cl 2/H 2O/Fe adlayer. Initial adsorption of Cl 2 results instead in Fe-Cl formation and weak interaction with adsorbed OH. The results emphasize the importance of solvation effects on impurity induced H 2O-surface interactions.

Friction studies in ultrahigh vacuum of Fe surfaces with thin films from exposure to perfluorodiethylether

The coefficient of friction for both clean and oxidized contacting polycrystalline Fe surfaces following exposure to a perfluorodiethylether (PFDE), (CF3CF2)2O, was measured in ultrahigh vacuum (UHV) using a pin-on-flat device (0.05–0.30 N loads). This system was chosen to examine the influence of thin films on friction using a molecule having functionalities that are present in perfluoropolyether lubricants. The friction coefficient measured for the clean (∼0.2–1.0 Å oxide) surfaces in UHV was ∼3.0. The friction was measured for both oxidized and clean Fe surfaces in PFDE atmospheres (3–150 Pa). During this exposure the friction for the oxidized Fe surfaces dropped to 0.7–0.9 compared to 1.2–1.5 obtained in O2 environments. The friction for clean Fe surfaces measured in UHV following similar PFDE exposures showed two distinct friction ranges, 0.1–0.8 and 1.2–1.8. The Fe surfaces were examined in situ in the same UHV chamber using x-ray photoelectron spectroscopy and showed the formation of a Fe fluoride film (8–16 Å). High resolution profiles of these surface scars using scanning tunneling microscopy in air revealed a contact morphology dependent upon the coefficient of friction. The lower-range friction scars had multiple point contacts while the higher-range scars exhibited single point contacts with deep surface ploughing.

Charge transfer effects on CO bond cleavage: CO and potassium on Fe(100)

The effects of submonolayer coverages of potassium on CO chemisorption on the Fe(100) surface has been studied with TPD, XPS, and He I and He II UPS. Thermal desorption experiments show an increased capacity of the surface to hold dissociated CO and a lowering of the energy barrier to dissociation. The weakly bound molecular CO states (α 1 and α 2) showed increased activation energies for desorption with increasing potassium coverage. However, the more strongly bound π-CO state (α 3) desorbs at a lower temperature with increasing potassium coverage. XPS data indicate that chemisorbed potassium on the clean Fe surface is bound in a semi-metallic state; not a fully ionic form. CO coadsorption partially oxidizes the chemisorbed potassium via a charge transfer mechanism. Potassium TPD data along with the K, C, O core level shifts demonstrate the formation of a K x O surface complex upon CO dissociation.

The Interaction of CFCl3 and CF2Cl2 with Solid Surfaces Studied by Thermal Desorption Mass Spectroscopy

AbstractAdsorption and desorption of CFCl3 and CF2Cl2 on Fe and pyrex glass surfaces has been studied in the temperature range 90–400 K. In contrast to a previous study of the interaction of CFCl3 with a clean Fe(100) surface, for the samples presently used no extra cleaning procedure has been applied in the UHV. Under these conditions adsorption only occurs at temperatures not far from conditions for condensation. From the desorption spectra, the activation energy of desorption for CFCl3 on a Fe surface is 36.8 ± 2 kJ mol−1 (at 148 K) and the heat of vaporization 34.7 kJ mol−1 (at 128 K). No evidence for dissociative adsorption of CF2Cl2 or CFCl3 (as was the case for CFCl3 on a clean Fe surface) has been found. In addition, no evidence for a photon stimulated decomposition of these molecules when adsorbed on these surfaces has been found when visible laser light is used. The results are discussed with respect to the problem whether surfaces may operate as sinks for halocarbons released into the atmosphere.

A combined XPS-UPS-EELS study of nitrogen adsorbed on clean and barium-promoted iron surfaces: The nature of the precursor to dissociation

For N 2 on a clean Fe surface, the adsorbed precursor in a parallel orientation becomes predominant around 110 K, while at lower temperatures it coexists with a weakly adsorbed species. On a Ba-promoted Fe surface, however, N 2 is present exclusively in the precursor state in the temperature range 80–150 K following moderate exposure. Besides exhibiting a low N-N stretching frequency of 1530 cm −1, the precursor shows a clear separation between the 5σ and 1π levels in the UPS; the precursor dissociates to give a nitridic species around 160 K.

Influence of preadsorbed oxygen on the sign and magnitude of the chemisorption-induced resistance change for H2 adsorption onto Fe films

Measurements have been made of the chemisorption-induced resistance change for H2 adsorbed onto Fe film substrates predosed with fixed coverages of chemisorbed oxygen. The measurements were made at temperatures from 295 to 340 K and for estimated oxygen coverages of less than 0.1 monolayers. Two distinct resistance change components were observed in both the adsorption kinetics and the equilibrium isotherms: a positive component which is associated with the adsorption of H2 onto a clean Fe surface, and a negative component which was correlated with the presence of the chemisorbed oxygen. The resistance change isotherms can be fit with a model which assumes that each of the resistance change components result from dissociative chemisorbed hydrogen. Possible mechanisms for the chemisorbed-oxygen-induced negative resistance change are discussed.

An auger and eels study of iron sulfide (FeS 2, F 7S 8 and FeS) surfaces

Auger and electron energy loss spectroscopy (EELS) were used to study the iron sulfide (FeS 2, Fe 7S 8 and FeS) surfaces. A characteristic M 2.3VV Auger doublet with a separation of 5.0 eV was observed on the Sulfides. An assignment of the electron energy loss peaks was made based on the energy dependence of the loss peaks and previous photoemission results. In addition to interband transitions and core level excitations, two volume plasmons and two surface plasmons were tentatively identified. The effect of argon ion bombardment was studied by monitoring the changes observed in the EELS spectra with sputtering time. Peaks with strong iron and sulfur character were observed. Heating the damaged sulfides results in reconstruction of the sulfide surfaces through migration of sulfur. For the undamaged sulfides, changes in the EELS spectra are due to variations in surface composition and crystallographic structure. For all the sulfides under study, some evidence of sulfur segregation to the surface was observed. Finally, the reactions of the sulfides with simple gases, such as H 2, CO and O 2 were studied. Heating with H 2 results in removal of elemental sulfur from the surface. CO reacts with Fe on the damage FeS 2 and Fe 7S 8 surfaces. The reaction products can be removed with H 2. All the sulfides are easily oxidized by oxygen to form α-Fe 2O 3. The reactions of H 2S and CO with clean Fe surfaces were also studied using EELS.

Formation and Decomposition of Nitrides on Iron Surfaces

Abstract The formation (by interaction with ammonia) and decomposition of nitrides on clean Fe surfaces was studied by means of Auger electron spectroscopy, x-ray photoelectron spectroscopy, thermal desorption spectroscopy, and scanning electron microscopy. The N atoms may exist in various forms with quite similar electronic properties, viz. as chemisorbed layer (= "surface nitride"), dissolved in α-Fe or γ-Fe, as γ′-nitride (= Fe4N) or as e-nitride, depending on temperature as well as pressure and duration of interaction with NH3. There is no noticeable chemical shift of the ionization energies of the Fe core levels, indicating that the bond is essentially covalent. The activation energy for the decomposition of e-nitride into Fe4N + N2 is about 27 kcal/mole, that for the decomposition of Fe4N into Fe+N2 ranges between 51 and 57 kcal/mole, depending on the mode of preparation. The latter values are identical to those found previously for the desorption of N2 from various Fe single crystal planes and indicate that the decomposition of the chemisorbed "surface nitrides" is the rate-limiting step which prevents the spontaneous decom-position of the metastable bulk iron nitrides.