Interaction of acetylene and ethylene with an Fe(111) surface

The chemisorption of C₂H₂ and C₂H₄ on a clean or partly C- or O-covered Fe(111) surface was investigated with AES, TDS and HREELS. On the clean surface, both molecules adsorb under strong rehybridization close to sp³. Above 230 K, C₂H₂ reacts to form CH and presumably CH₂ as the main products, which on further heating decompose to yield H₂ desorption maxima at 580 and 490 K, leaving two carbon species on the surface which correspond to two loss peaks at 400 and 1290 cm^?1 in the HREELS spectrum. C₂H₄ undergoes very rapid decomposition above 250 K; no intermediates have been detected. The presence of coadsorbed oxygen or carbon atoms only reduced the maximum uptake of C₂H₂, but led to the appearance of new molecular adsorption states of C₂H₄ and inhibited C₂H₄ decomposition.     

The interaction of hydrazine with an Fe(111) surface

The interaction of hydrazine with a clean and nitrogen precovered Fe(111) surface was investigated in the temperature range of 126–600 K by means of UV and X-ray photoelectron spectroscopy (PES). At temperatures below 170 K the molecular adsorption of hydrazine is followed by multilayer condensation. In going from adsorbed to condensed hydrazine the valence and core levels shift in different directions relative to the vertical gas phase ionisation energies indicating strong interactions via hydrogen bonding in the condensed phase. Dissociative adsorption of N₂H₄ was observed at temperatures above 220 K. At room temperature no difference in the photoelectron spectra following the adsorption of N₂H₄ or NH₃ was observed indicating the presence of the same surface species, predominantly being -NH₂ radicals. Preadsorbed nitrogen stabilizes N₂H₄ against decomposition. The results will be discussed in view of possible intermediates in the ammonia-synthesis reaction on iron. Simple thermochemical arguments are presented to explain the observed difference in the heterogeneous dissociation mechanism of hydrazine on transition metals. General conclusions on the mechanism of ammonia synthesis on various transition metals can also be derived from these thermodynamic considerations.     

Interaction of carbon dioxide with Fe(110), stepped Fe(100) and Fe(111)

The adsorption of CO₂ on single crystal surfaces of Fe(110), regularly stepped Fe(110) and Fe(111) in the temperature range between 77 and 340 K was studied by means of He(I) UPS and measurements of the change in work function. The smooth Fe(110) face proved to be completely inactive with respect to CO₂ adsorption. On a stepped Fe(110) and an Fe(111) face CO₂ is adsorbed at 77 K in the form of a linear molecule and in the form of a species the nature of which is not yet clarified. This latter form is predominant at 140 K. With increasing temperature decomposition into CO and O and finally into C and O takes place.     

Interaction of oxygen with a Mo(111) surface

Oxygen adsorption on a Mo(111) surface is investigated at low pressures (10^?7 to 10^?5 Pa) and room temperature by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). In agreement with previous studies it is established that the surface is not reconstructed during adsorption and the oxygen forms no ordered structures. On the basis of kinetic and spectroscopy data, the formation of two adsorption states on the surface within 1 monolayer is established. The valence band of a clean surface is studied in detail. An attempt is made to ascribe the peaks obtained to definite d states. The interaction between O₂ and Mo(111) is discussed in terms of the results obtained and a comparison with the O₂/W(111) system is made.     

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NO_ad. The mean adsorption energy is about 25 $\text{kcal}\text{mole}$. Dissociation of adsorbed NO starts already at ?120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N₂ occurs around 600 °C with an activation energy of about 52 $\text{kcal}\text{mole}$. A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of N_ad + O_ad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H₂. This yields an N_ad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ ?2, ?8.8 and ?14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. N_ad as well as O_ad causes a peak at ?5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.     

Effect of impurities on the surface morphology of Fe(111)

The surface composition and morphology of Fe(111) have been examined through a combined analysis that includes low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM). The preferential segregation of sulfur has been clearly identified by AES upon annealing. The STM images exhibit numerous triangular pits of various sizes, and the LEED patterns show diffused n × 1 spots. The triangular pits reveal a Sierpinski gasket fractal. For sulfur-free Fe(111), nitrogen segregates to the surface upon annealing, forming a 4√3 × 4√3 superstructure that is identified by LEED patterns and STM images. The STM images show nanoscale triangular clusters regularly aligned in a hexagonal 4√3 × 4√3 configuration. Ultra-thin chromium film deposited on a nitrogen-segregated Fe(111) surface with post-annealing induces further nitrogen segregation, resulting in the formation of triangular pyramid-shaped CrN nanoclusters.     

Interaction of CO with Cu(111)-Fe

The interaction of CO with a Cu(111) crystal alloyed with different amounts of iron is studied with ellipsometry and AES. Both molecular and dissociative adsorption are observed. The final coverages of oxygen and carbon are equal and such that the sum is the same or larger than the mole fraction of iron in the surface layer. The amount of molecularly adsorbed CO does not increase when a quantity of more than about four monolayers of iron is deposited. This is most probably due to the fact that the iron enrichment of the outer surface layer ceases as well. The isosteric heat of adsorption of CO on the Cu(111)-Fe crystal is 70±15 kJ/mol, independent of coverage and iron content of the surface within experimental error.     

Interaction of HNCO with Au(111) surfaces

The surface chemistry of isocyanic acid, HNCO, and its dissociation product, NCO, was studied on clean, O-dosed and Ar ion bombarded Au(111) surfaces. The techniques used are high resolution energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The structure of Ar ion etched surface is explored by scanning tunneling microscopy (STM). HNCO adsorbs molecularly on Au(111) surface at 100 K yielding strong losses at 1390, 2270 and 3230 cm^? 1. The weakly adsorbed HNCO desorbs in two peaks characterized by T_p = 130 and 145 K. The dissociation of the chemisorbed HNCO occurs at 150 K to give NCO species characterized by a vibration at 2185 cm^? 1. The dissociation process is facilitated by the presence of preadsorbed O and by defect sites on Au(111) produced by Ar ion bombardment. In the latter case the loss feature of NCO appeared at 2130 cm^? 1. Isocyanate on Au(111) surface was found to be more stable than on the single crystal surfaces of Pt-group metals. Results are compared with those obtained on supported Au catalysts.     

The oxidation of Fe(111)

The oxidation of Fe(111) was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and scanning tunnelling microscopy (STM). Oxidation of the crystal was found to be a very fast process, even at 200 K, and the Auger O signal saturation level is reached within ~ 50 × 10^? 6 mbar s. Annealing the oxidised surface at 773 K causes a significant decline in apparent surface oxygen concentration and produces a clear (6 × 6) LEED pattern, whereas after oxidation at ambient temperature no pattern was observed. STM results indicate that the oxygen signal was reduced due to the nucleation of large, but sparsely distributed oxide islands, leaving mainly the smooth (6 × 6) structure between the islands. The reactivity of the (6 × 6) layer towards methanol was investigated using temperature programmed desorption (TPD), which showed mainly decomposition to CO and CO₂, due to the production of formate intermediates on the surface. Interestingly, this removes the (6 × 6) structure by reduction, but it can be reformed from the sink of oxygen present in the large oxide islands simply by annealing at 773 K for a few minutes. The (6 × 6) appears to be a relatively stable, pseudo-oxide phase, that may be useful as a model oxide surface.     

硫在Fe(111)面吸附的密度泛函研究

采用基于密度泛函理论的第一性原理方法,系统研究了不同覆盖度下硫在Fe(111)表面的吸附构型和吸附特性,计算并分析了硫在Fe(111)表面的吸附能、电荷密度、分波态密度、电荷布局、电子局域化函数等数据.研究结果表明:S在Fe(111)面的H位吸附最稳定,并且吸附能随着覆盖度的增加而增加.另外,电子态密度、电子局域化函数和布局分析表明Fe、S之间呈较弱的共价键,这种作用力主要是Fe的3d轨道和S的3p轨道杂化所贡献,而随着覆盖度的增加,Fe、S之间的作用力逐渐减弱,这可能是由于S原子之间的排斥力减弱了Fe、S之间的作用.S在Fe(111)、Fe(110)和Fe(100)这三个晶面上吸附情况的对比分析发现,S与Fe(111)表面的相互作用最强,Fe(100)面次之,而Fe(110)面最弱.     

Diffusion of atoms on an fcc(111) surface

The motion of adatoms on an fcc(111) surface is modeled. In light of recently revealed specific features of the potential energy surface, a new empirical potential taking into account the nonequivalence of fcc and hcp adsorption sites was proposed. The modeling was performed by the method of stochastic cycles, a new variety of MDM. Calculations were performed for the diffusion of oxygen atoms over a platinum surface. The potential energy surface was constructed based on experimental data and DFT calculations. An analysis demonstrated that the diffusion activation energy coincides with the static barrier while the preexponential factor is weakly temperature-dependent. Note, however, that the degree of asymmetry of the potential produces a significant effect on the preexponential factor: the changeover from a symmetric two-well potential to a strongly asymmetric single-well potential causes an increase in the preexponential factor by an order of magnitude.