Sulfur Overlayers Research Articles

Disordered asymmetrical surface structures of clean Mo(100)-“(1 × 1)” and of “c(2 × 2)” sulfur on Mo(100) from dynamical leed analyses

The structures of the clean “(1 × 1)” and the “c(2 × 2)” sulfur-covered molybdenum (100) surfaces have been studied by low-energy electron diffraction (LEED) intensity analyses. It is found that the atoms in the top layer of both these surfaces probably reside at asymmetrical adsorption sites, destroying the perfect (1 × 1) and c(2 × 2) periodicities normally deduced from the LEED patterns. For clean room-temperature Mo(100), a slight preference is found for the topmost atoms to be displaced 0.13 ± 0.05 Å away from the centers of hollow sites of the second metal layer. This displacement occurs in the four equivalent 〈11〉 surface directions (diagonally in the square surface lattice), possibly randomly. The clean surface exhibits first, second and third interlayer spacing relaxations of − 6% ± 1.5%, 2% ± 2% and 0.5% ± 3%, respectively, relative to the bulk interlayer spacing. In the “c(2 × 2)” sulfur overlayer, the sulfur atoms are found to reside away from the center of hollow sites by 0.20 ± 0.05 Å. This displacement occurs in the four equivalent 〈10〉 surface directions (parallel to the sides of the square metal lattice cells), possibly randomly. The sulfur-Mo interlayer spacing is 1.03 ± 0.02 Å, while the spacings between metal layers have relaxed closer to their bulk value, which a top metal-metal spacing still contracted by 3% ± 3%. The asymmetrical site gives rise to three different bond lengths between sulfur and molybdenum atoms: 2.33 and 2.58 Å for Mo atoms in the topmost metal layer, and 2.56 Å for Mo atoms in the second metal layer. In this case, no lateral Mo relaxations were investigated.

An XPS investigation of the surface of natural sphalerites under flotation-related conditions

The surface composition of two natural sphalerites of different iron and lead content has been determined under flotation-related conditions by X-ray photoelectron spectroscopy (XPS). Oxidation of the mineral samples in air-saturated alkaline solutions has been found to be very slow. In the presence of copper (II) species in solution, copper exchanges with zinc, and metal-deficient environments are formed in the sulfide lattice at the surface. Cyanide removes the copper and some sulfur from the surface, but a metal-deficient sulfide layer remains. Treatment of sphalerite with cyanide can also result in lead impurity in the mineral migrating to the surface to form lead cyanide and hydrated lead oxide. Zinc-oxygen species were detected at the surface of sphalerite that had been immersed in alkaline zinc solutions and washed with dilute sodium hydroxide before examination. Copper was found to diffuse to the surface when sulfur was deposited on sphalerite by air oxidation of hydrosulfide ions. The resulting surface was a metal-deficient sulfide rather than an overlayer of elemental sulfur.

Thiophene hydrodesulfurization over rhenium surfaces: the effects of structure and adsorbates (carbon and sulfur)

Thiophene hydrodesulfurization (HDS) has been investigated over rhenium single crystals and polycrystalline foils and was found to be a structure-sensitive reaction over these surfaces. Rhenium single crystals are 1-6 times more active than molybdenum single crystals, which is in agreement with previous studies using rhenium disulfide and molybdenum disulfide catalysts. Adsorbed carbon and sulfur overlayers decrease the activity of the rhenium single crystals, suggesting that the catalyst surface remain free of strongly bound deposits of carbon and/or sulfur. The trend in HDS activity measured for the Re catalyst can be explained by considering the difference in coordination of the top layer rhenium atoms in the single-crystal surfaces.

STM study of the structure of the sulphur (1×2) overlayer on molybdenum (001) in air: ordered domains, phase boundaries and defects

SUMMARYA (001) surface of molybdenum, covered by one monolayer of sulphur was prepared in UHV and characterized by LEED, Auger and XPS. This surface was found to be stable in air for periods of several days. STM images of the surface, obtained in air in the topographic and local barrier height modes, revealed the atomic arrangement of sulphur atoms in domains with 1×2 and 2×1 periodicities. Boundaries between domains, adsorbate and substrate defect structures and crystallites formed during the initial oxidation of the Mo substrate were observed.

The adsorption and coadsorption of sulfur and carbon monoxide on rhenium single crystal surfaces

The structure of sulfur overlayers on Re(0001) and Re(10 10) , and their effect upon CO chemisorption have been studied using Auger electron spectroscopy, thermal desorption spectroscopy, and low energy electron diffraction. In addition, calculations based upon a kinematic analysis of electron scattering have yielded possible two-dimensional (2D) real space structures based upon the observed LEED patterns for sulfur adsorbed on Re(0001). Four sulfur overlayer structures were seen on Re(0001): starting with a p(2×2) at 0.25 monolayers ( θ s = 0.25), a “ (3 3 ×3 3 ) R30° ” with missing spots at θ s = 0.35, a 3 1 1 3 at θ s = 0.4, and a (2 3 ×2 3 ) R30° at θ s = 0.5, with saturation occurring at approximately θ s ≅ 0.6. Two sulfur structures are observed on Re(10 10) , a p(2×2) at θ s = 0.5, and a p( l ×2) at θ s = 1.0. Desorption energies for sulfur on both surfaces were determined to range from 104 kcal/mol at low sulfur coverage to 85 kcal/mol near saturation. Comparisons with calculations based upon a kinematic analysis of the high sulfur coverage diffraction patterns on Re(0001) suggest that close proximity between adsorbed sulfur atoms may be responsible for lowering the adsorption energy. On Re(10 10) , an adsorption site for sulfur is proposed which involves first and second layer atoms. CO chemisorption was inhibited linearly by increasing the concentration of pre-adsorbed sulfur. A slight decrease in the CO desorption peak temperature at low sulfur coverage on Re(0001) was also observed. Comparison with other systems such as Ru(0001) enables us to conclude that the decrease in peak temperature is due to repulsive interactions between CO and S. On the other hand, pre-adsorbed sulfur on Re(10 10) blocks CO desorption peaks at low temperature, suggesting that pre-adsorbed sulfur blocks CO-CO repulsion.

Anharmonic effects on adsorbate vibrations

We have measured the temperature dependence of the frequency and the line width of the perpendicular adsorbate mode for the c(2 × 2)- and p(2 × 2)-oxygen and the c(2 × 2)-sulfur overlayer on Ni(100). The frequency of the perpendicular adsorbate mode decreases linearly with temperature. The line width shows a strong temperature dependence. At 300 K the line width of the perpendicular sulfur mode is 22 cm −1. For the p(2 × 2)-oxygen structure, a line width of 26 cm −1 was found. The line width of the perpendicular oxygen vibration of the c(2 × 2)-oxygen overlayer is 60 cm −1. Around 300 K, the line width depends linearly on temperature. The line width can be explained by two phonon processes and an additional inhomogeneous broadening.

The Deposition of Sulfur on Pyrite and Chalcopyrite From Sodium Sulfide Solutions

The deposition of sulfur from oxygenated sodium sulfide solutions on to fracture surfaces of massive natural pyrite from two different locations has been studied by X-ray photoelectron spectroscopy. For both samples an overlayer of elemental sulfur was formed from sulfide concentrations between 10-5 and 5×10-2 mol dm-3 with a maximum near 10-2 � mol dm-3; the decrease in sulfur coverage at the higher concentrations is accounted for by polysulfide formation. However, under the same conditions, the sulfur coverage on pyrite from Spain was double that on pyrite from Elba. Although the two minerals contain different levels of trace elements, it was not possible to identify the cause of the difference in behaviour. The incorporation of copper into the surface of the Spanish pyrite decreased the sulfur coverage by a factor of two, indicating that foreign atoms can have a significant effect. The sulfur layer formed on chalcopyrite after immersion in sodium sulfide solutions was about half that on Spanish pyrite treated simultaneously. The significance of these findings to the selective collectorless flotation of chalcopyrite/pyrite ores is discussed.

Adsorption of H2S on Ni(100): Kinetics and structure

The kinetics of H 2 S adsorption on Ni(100) is studied at various temperatures, using X-ray photoelectron spectroscopy and low energy electron diffraction. The variation of the sticking coefficient with sulphur coverage is explained by computer simulation. The simulation is based on a precursor picture of the adsorption process and further includes the formation of antiphase boundaries in the sulphur overlayer. A change in the long range order of the overlayer is observed to take place at 600 K.

Comment on ‘on the structure of chemisorbed sulphur overlayers on Ni(100)’ by W. Daum

Abstract The work of Daum [Surface Sci. 182 (1987) 521] on the long range order of the p(2 × 2) superstructure of S on Ni(100) is commented upon. The experiments of Daum have been extended to other temperatures and it is concluded that a perfect p(2 × 2) structure of S on Ni(100) does exist and that annealing of the sulphur layer at temperatures above 600 K leads to the formation of the streaky LEED patterns reported by Daum.

Comment on ‘on the structure of chemisorbed sulphur overlayers on Ni(100)’ by W. Daum

The work of Daum [Surface Sci. 182 (1987) 521] on the long range order of the p(2 × 2) superstructure of S on Ni(100) is commented upon. The experiments of Daum have been extended to other temperatures and it is concluded that a perfect p(2 × 2) structure of S on Ni(100) does exist and that annealing of the sulphur layer at temperatures above 600 K leads to the formation of the streaky LEED patterns reported by Daum.

Reply to “comment on ‘on the structure of chemisorbed sulfur overlayers on Ni(100)’” by J.N. Andersen

Reply to “comment on ‘on the structure of chemisorbed sulfur overlayers on Ni(100)’” by J.N. Andersen

Reply to “comment on ‘on the structure of chemisorbed sulfur overlayers on Ni(100)’” by J.N. Anderson

Reply to “comment on ‘on the structure of chemisorbed sulfur overlayers on Ni(100)’” by J.N. Anderson

Surface phonon dispersion of Cu(100)-p(2×2)S

The dispersion of adsorbate and substrate associated modes of the Cu(100) surface covered with a p(2×2) sulfur overlayer has been studied along the ¯ΓX and the ¯ΓM direction by inelastic electron scattering. The Rayleigh mode (S4 in the ¯ΓX and S1 in the ¯ΓM direction) is only slightly changed by the sulfur overlayer. One adsorbate-induced phonon branch above the bulk phonon band is fond. This mode shows a dispersion of 35 cm−1 in both directions and is attributed to the perpendicular vibration of the adatoms. Lattice dynamical calculations show that the parallel vibration mode has no dispersion throughout the two-dimensional Brillouin zone. For a perpendicular distance of 1.39 Å of the sulfur atoms to the first Cu layer the frequency of the parallel and perpendicular mode is almost the same (300 cm−1) in the middle of the Brillouin zone.

H 2S/Cu(111): A model study of sulfur poisoning of water-gas shift catalysts

The interaction of H2S with Cu(111) has been studied with XPS, AES, ISS and LEED; and the effects of sulfur adatoms upon the kinetics of the medium-pressure water-gas shift reaction over Cu(111) have been measured. At 120 K, H2S adsorption is partially molecular. By 200 K, some of the H2S desorbs and the rest dissociates to produce sulfur adatoms bonded on top of the Cu(111) surface. The surface saturates at room temperature in a (7x7) structure (θs=0.43) characteristic of a nearly close-packed sulfur overlayer, very similar to layers in (111)-oriented Cu2S. Sulfur adatoms are quite stable to heating in vacuum or H2, with a reaction probability for removal per H2 collision with the surface of ≤10−9 at 636 K. The rate of the forward water-gas shift reaction (H2O+CO→H2+CO2) decreases linearly with sulfur coverage as (1–2.6θs). This poisoning is attributed to steric blocking by the sulfur adatoms of the sites required for dissociative water adsorption. The site ensemble required for this step is small (1–2 Cu atoms). The well-known sensitivity of shift catalysts to sulfur poisoning is discussed in the light of these results.

On the structure of chemisorbed sulfur overlayers on Ni(100)

On the structure of chemisorbed sulfur overlayers on Ni(100)

On the structure of chemisorbed sulfur overlayers on Ni(100)

A quarter of a monolayer of sulfur adsorbed on Ni(100) does not form a well-ordered p(2 × 2) overlayer as reported by other authors. In agreement with Hagstrum and Becker the diffraction pattern always showed weak streaks between the superstructure spots and indicates considerable amounts of (2 × 1) domains. Like for the c(2 × 2)S surface (θ s = 0.5) the frequency of the dipole active sulfur vibration is 350 cm −1 also for θ s = 0.25.

A UPS and XPS investigation of the adsorption of sulfur on Mo(100): Adsorption site and lateral interactions

UV and X-ray photoemission spectroscopies have been used to study the sulfided Mo(100) surfaces, augmenting existing LEED, AES, TDS and work-function results. The results show that photoelectron spectroscopy is capable of distinguishing sulfur atoms chemisorbed in different sites at the Mo(100) surface by careful examination of the binding energies of the adlayer photoemission peaks. A model for the adsorbate-adsorbate interaction within the ordered sulfur overlayers is used to discuss the structure of these overlayers. At sulfur coverages 2 is shown to require diagonally adjacent four-fold hollow sites.

The adsorption and reactions of hydrocarbons on Molybdenum single crystal surfaces; when clean and in the presence of co-adsorbed sulfur or carbon

The chemisorption and reactions of thiophene, 1,3-butadiene, butenes, and n-butane on clean Mo(100), and with sulfur or carbon overlayers, have been investigated using thermal desorption spectroscopy (TDS). The predominant reaction at low additive coverage (0–0.2 monolayers of S or C), and at low ambient pressure (10 -10 Torr) for unsaturated hydrocarbons is decomposition. Butane is the least reactive of the hydrocarbons, and a large fraction (95%) desorbs molecularly at all additive coverages. As additive (S or C) coverage increases the amount of decomposition decreases, enabling other reaction pathways to become more probable. Hydrogenation, partial dehydrogenation, and isomerization reactions are detected. Molecular binding on the additive overlayers was also found to be very different. On sulfur overlayers the binding of the hydrocarbons was weak (physisorption), usually on the order of the heat of sublimation (9–10 kcal/mol). However, molecular binding on carbon overlayers was stronger: the heat of desorption was 17–23 kcal/mol for thiophene and butadiene, 12–15 kcal/mol for the butenes, and 11 kcal/mol for butane. In addition, isomerization of 1-butene to 2-butene, occurred on the carbon overlayer. It is suggested that the metal sites control the reactions observed (except for isomerization). This accounts for the similarity in surface reaction product distributions, and explains why the difference in molecular hydrocarbon binding between the sulfur covered and carbon covered surface plays a minor role in these reactions.

Coadsorption of sulphur and hydrogen on Pt(111) studied by radiotracer and electrochemical techniques

The influence of chemisorbed sulphur on the adsorption of hydrogen on Pt(111) in aqueous acid medium was quantitatively studied. Isotherms of electroadsorption of hydrogen on Pt(111) covered with various amounts of sulphur are reported. The relation between the coverages of sulphur and hydrogen coadsorbed on the surface was established. The adsorption of hydrogen is totally inhibited by a sulphur coverage well below saturation coverage. The θ H versus θ S curve is not linear. At low θ S, θ H decreases sharply with increasing θ S. The number of hydrogen adsorption sites blocked by one sulphur atoms is 8 ± 1. A model of nearest-neighbour sites poisoning fits well this experimental value. The same model of blocking also fits well the experimental variation up to θ S ∼ 0.3, assuming a random distribution of sulphur. Above, the rapid fall-of of θ H to zero is best explained by a model of adsorption of hydrogen in an ordered sulphur overlayer structure. The shape of the isotherms indicates that sulphur weakens the Pt-H bond and reduces the interactions between H atoms.

Coadsorption of sulphur and hydrogen on Pt(111) studied by radiotracer and electrochemical techniques

The influence of chemisorbed sulphur on the adsorption of hydrogen on Pt(111) in aqueous acid medium was quantitatively studied. Isotherms of electroadsorption of hydrogen on Pt(111) covered with various amounts of sulphur are reported. The relation between the coverages of sulphur and hydrogen coadsorbed on the surface was established. The adsorption of hydrogen is totally inhibited by a sulphur coverage well below saturation coverage. The θ H versus θ S curve is not linear. At low θ S, θ H decreases sharply with increasing θ S. The number of hydrogen adsorption sites blocked by one sulphur atoms is 8±1. A model of nearest-neighbour sites poisoning fits well this experimental value. The same model of blocking also fits well the experimental variation up to θ S≈0.3, assuming a random distribution of sulphur. Above, the rapid fall-of of θ H to zero is best explained by a model of adsorption of hydrogen in an ordered sulphur overlayer structure. The shape of the isotherms indicates that sulphur weakens the Pt-H bond and reduces the interactions between H atoms.