The interaction of oxygen and carbon monoxide with a carbided Ni(111) surface

The thermal decomposition of ethylene on Ni(111) at 250°C is shown to lead to carbon deposition on and - in a later stage - below the surface. Independent of the amount of carbon below the surface, CO is adsorbed with an isosteric heat of adsorption of 105 kJ/mol. The surface carbon reacts with oxygen at 250°C. The reaction rate is independent of the surface carbon coverage and first order in oxygen pressure. The subsurface carbon segregates to the surface after removal of the surface carbon layer.     

The interaction of oxygen with aluminium (111)

The initial uptake of oxygen by an aluminium (111) surface at 300 K has been studied by several experimental techniques including XPS, AES, Δφ, ellipsometry and surface plasmon spectroscopy. The interaction falls into two stages. Oxygen exposure up to 30–50 L in the first stage results in a chemisorbed layer with oxygen atoms located in the immediate surface region. In the second stage increased exposure produces a thin, relatively homogeneous oxide film.     

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.     

The co-adsorption of oxygen and hydrogen on Rh(111)

The co-adsorption of oxygen and hydrogen on Rh(111) at temperatures below 140 K has been studied by thermal desorption mass spectrometry, Auger electron spectroscopy, and lowenergy electron diffraction. The co-adsorption phenomena observed were dependent upon the sequence of adsorption in preparing the co-adsorbed overlayer. It has been found that oxygen extensively blocks sites for subsequent hydrogen adsorption and that the interaction splits the hydrogen thermal desorption into two states. The capacity of the oxygenated Rh(111) surface for hydrogen adsorption is very sensitive to the structure of the oxygen overlayer, with a disordered oxygen layer exhibiting the lowest capacity for hydrogen chemisorption. Studies with hydrogen pre-adsorption indicate that a hydrogen layer suppresses completely the formation of ordered oxygen superstructures as well as O₂ desorption above 800 K. This occurs with only a 20% reduction in total oxygen coverage as measured by Auger spectroscopy.     

A LEED/UPS study on the interaction of oxygen with a Ni(111) surface

Exposure of a Ni(111) surface to oxygen leads at first to the formation of a chemisorbed overlayer which is characterized by a 2 × 2-superstructure and a maximum in the photoemission spectrum (hv = 40.8 eV) centered at 5.6 eV below the Fermi level E_F. The emission from the Ni d-states is nearly unaffected at this stage of interaction. After high oxygen exposures the epitaxial growth of NiO can be identified from the LEED pattern. The corresponding photoelectron spectrum is strongly altered and exhibits close agreement with the transition energies as calculated by Messmer et al. for a NiO₆^10- -cluster.     

The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

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.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

Photoemission study of the chromium(111) surface interacting with oxygen

The interaction of the Cr(111) surface with O₂ was studied by means of X-ray and UV photoemission and also work function measurements. A strong oxygen adsorption was found even at very low exposures, suggesting a high sticking coefficient. Previous treatments of the clean surface such as argon-ion bombardment or annealing result in significant changes of the surface structure reflected on work function and adsorption kinetics. No work function change was observed in the initial stage of adsorption, ruling out a model of chemisorption on top. In this range the sticking coefficient remains also constant, supporting a model of rapid regeneration of the genuine surface sites and incorporation of oxygen into the lattice. But in contrast with non transition metals like Cs or Sr, oxygen absorbed at room temperature in Cr, remains essentially in the topmost layers of the surface. At room temperature this initial stage of oxygen incorporation is followed by chemisorption on the corrosion film obtained when the uppermost layers are saturated with oxygen. The oxide layer has a stoichiometry close to Cr₂O₃ at saturation, but the detailed electronic structure depends on previous thermal treatments. Exposures at room temperature lead to a thin (about 9 Å), probably amorphous corrosion layer with a maximum work function change Δφ = +0.9 eV. Adsorption followed by heating at 500° C results in a much thicker corrosion film with a limiting work function decrease of Δφ = ?1.2 eV. The XP and UP spectra differ significantly in both cases and suggest a Fermi level shift of nearly 1 eV connected with oxygen adsorption on the Cr₂O₃ surface. The thickness of the corrosion film may be further increased by heating at 500°C in oxygen. The usual XPS spectra of bulk chromium sesquioxide are then clearly observed.     

Properties of the interaction of europium with Si(111) surface

Formation of the Eu/Si(111) system as the metal layer thickness gradually increases from 0.5 to 60 monolayers (ML) deposited on the silicon surface at room temperature, and after heating at up to 900 °C, has been studied by Auger electron spectroscopy, electron-energy-loss spectroscopy, and low-energy-electron diffraction. It is shown that room-temperature film growth passes through three stages, depending on the Eu layer thickness: metal chemisorption, interdiffusion of the metal and substrate atoms, and buildup of the metal on the surface of the system. Heating of ultrathin (about one ML) Eu films deposited at room temperature results in ordering of metal atoms on the silicon surface with only weak interaction. Heating thick (above 15 ML) Eu layers on the silicon surface produces silicides whose structure depends on the heating temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 562–567 (March 1998)     

Chemisorption of oxygen on the silver (111) surface

The interaction of oxygen with the (111) surface of a silver single crystal is studied, mainly in the pressure range from 10^?3 up to 1 torr and at temperatures from room up to 500°C. The experimental techniques employed were LEED, secondary electron spectroscopy, work function variation measurements, and desorption kinetics. Exposure to the high pressures was made with a sample isolation valve. The experimental procedures are examined in detail and critically discussed. The results obtained with the different techniques allow a correlation with many studies of other authors. The LEED technique indicates that in the range of pressures and temperatures examined, a surface superstructure is stable, having a unit mesh with sides four times greater than that of the silver (111) plane. The presence of this surface phase seems to be related to oxygen adsorbed in the dissociated form. On this assumption, an interpretation of the structure is proposed, which is based on a coincidence lattice formed by a (111) plane of Ag₂O on the (111) plane of the metal. This interpretation is also in agreement with the thermodynamic data.     

On the kinetics of oxygen adsorption on a Pt(111) surface

The kinetics of O₂ adsorption on a clean Pt(111) surface were investigated in the temperature range 214–400°C. The oxygen coverage was measured by CO titration as well as Auger electron spectroscopy both of which show the same dependence on O₂ exposure. The initial sticking coefficient on clean Pt(111) is 0.08–0.10 and decreases exponentially with increasing oxygen coverage. For θ > 0.23 a (2 × 2)-O LEED pattern was observed. The highest oxygen coverage obtained was approximately 0.45. A theoretical model was proposed which correlates the coverage dependence of the sticking coefficient with adsorbate interactions in the chemisorbed state. These interactions cause a coverage dependent activation energy of adsorption assuming the existence of a precursor state. Experiments dealing with the effect of carbon contamination on the sticking coefficient showed that the initial sticking coefficient decreases with increasing carbon coverage.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.