Adsorption of SH and OH and coadsorption of S, O and H on Ni(111)

The adsorption of SH and OH radicals on Ni(111) is treated using an ab initio embedding theory. The Ni(111) surface is modeled as a three-layer, 28-atom cluster with the Ni atoms fixed at bulk lattice sites. The Ni(111) energy surface is very flat for SH adsorption if the H tilt angle is allowed to vary. At both atop and bridge sites, the S---H axis is tilted away from the surface normal by 70°, resulting in the sulfur atom being sp³-hybridized and the adsorption energy being 59 kcal mol⁻¹. For SH at the three-fold site, the S---H axis is normal to the surface, the sulfur is sp-hybridized, and the adsorption energy is 58 kcal mol⁻¹. OH is preferentially adsorbed at the three-fold site. The calculated adsorption energy is 90 kcal mol⁻¹ and the O---H axis is perpendicular to the surface. OH adsorption at the atop and bridge sites is 16 and 5 kcal mol⁻¹ less stable than at the three-fold site, respectively. Atomic H, O and S are preferentially adsorbed at the three-fold site. The calculated adsorption energies are 62, 92 and 87 kcal mol⁻¹, for H, O and S, respectively. The calculated adsorbate---Ni bond distances of 1.86 Å for H, 1.86 Å for O and 2.29 Å for S are in good agreement with experimental data. SH and OH bonding to the surface involves a combination of ionic and covalent contributions and substantial mixing with the Ni 3d orbitals. Dipole-moment calculations indicate strong ionic bonding for the atomic O/Ni system and ionic plus covalent character for the atomic S/Ni interactions. Adsorption of S and O at the three-fold site blocks H adsorption at the nearby surface. Moving H away from the S or O adatom reduces the repulsion. The dissociation of SH_ad → S_ad + H_ad is calculated to be exothermic by 5 kcal mol⁻¹ and OH_ad → O_ad + H_ad to be endothermic by 30 kcal mol⁻¹ for infinite separation between S, O and H.     

Adsorption position of oxygen on the Pt(111) surface

The adsorption position of oxygen on the clean Pt(111) surface has been determined by means of the transmission channeling technique. Oxygen adsorbs in a well ordered p(2 × 2) overlayer structure at temperatures 200 T 350 K. From an analysis of the angular scans along the [111], [110] and [100] axial directions it is concluded that the O atoms are adsorbed in the fcc three-fold hollow site exclusively at a height of 0.85 ± 0.06 Å above the Pt surface layer. From a narrowing of the [111] angular O scan, the O RMS displacement parallel to the surface is found to be 0.16 ±0.03 Å.     

Surface structures of S/Ni(111) in (√3 × √3 )R30° and (5√3 × 2) phases studied by surface extended X-ray absorption fine structure spectroscopy

The local surface structures of S/Ni(111) in the ( √3 × √3) R30° and (5√3 × 2) phases have been investigated by means of polarization-dependent sulfur K-edge surface EXAFS. In the (√3 × √3 ) R30° phase, sulfur adatoms are found to occupy threefold hollow sites with a S---Ni distance of 2.13 Å and an inclination angle ω of the Sz.sbnd;Ni bonds at 44° from the surface plane. In contrast, in the (5√3 × 2) phase, it is determined that the Sz.sbnd;Ni bond is longer, 2.18 Å, more inclined, ω = 31°, and that the coordination number is not 3 but 4. These results strongly support a picture involving reconstruction of the top nickel layer to form a rectangular structure. Consideration of several models proposed for the (5√3 × 2) phase leads to one which is compatible with both the present results and results recently reported using STM.     

Structural, reactive and kinetic properties of acetylene on clean Mo(100)

Acetylene adsorbs onto Mo(100) at 80 K via precursor state kinetics, saturating the chemisorbed overlayer at a coverage of 1.0. Further exposure to acetylene adsorbs multilayers, which are desorbed by heating to 100 K. Two surface species are identified using angle-resolved ultraviolet photoelectron spectroscopy after adsorbing acetylene at 80 K. One species is undistorted chemisorbed acetylene, the other is an acetylenic species which is rehybridized to approximately sp² and is adsorbed with its C-C axis oriented at 45° with respect to the surface normal. On heating this surface to above 110 K, all the chemisorbed acetylene converts into the rehybridized species. This is stable up to a surface temperature of 180 K, following which it starts to decompose. Thermal decomposition is complete by 300 K. Only hydrogen is detected in thermal desorption spectroscopy, and the spectra are characteristic of hydrogen desorption from a carbided surface.     

Nickel carbonyl adsorption on the Ni(111) surface

Overlayers formed by the adsorption of Ni(CO)₄ in CO on the Ni(111) surface at 100 K were characterized using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At temperatures below 135 K, molecular nickel carbonyl adsorbs on the CO saturated Ni(111) surface as suggested by several observations. Vibrational transitions characteristic of molecular Ni(CO)₄ are dominant. The energy dependence of both the elastic and inelastic electron scattering cross sections are dramatically altered by Ni(CO)₄ adsorption. All of the mass spectrometer ionization fragments typical of molecular Ni(CO)₄ are observed in the narrow thermal desorption peak at 150 K. The inelastic scattering cross sections for both adsorbed nickel carbonyl and adsorbed CO on the Ni(111) surface suggest that a nonresonant dipole scattering mechanism is dominant.     

Four-fold hollow site for S in a Ni(100) raft on NiO(100)

The reaction of H₂S with NiO(100) has been studied by polarization-dependent surface EXAFS. The results evidence reduction of the selvedge to form a Ni raft having S in four-fold sites with a S–Ni bond length of 2.21±0.02 Å. The Ni–Ni in-plane distance is 2.77±0.09 Å, representing a 6±4% contraction compared to that in NiO(100).     

Determination of the K adsorption site on Fe(1 1 0) with XSW

We have used X-ray standing waves (XSW) in near normal incidence to determine the K–Fe bond length and the adsorption site of K at the saturation coverage at room temperature on the Fe(1 1 0) surface. Three different scattering geometries were used to enable the determination of the adsorption site by triangulation. From the results we conclude that the potassium atoms adsorb in a distorted hexagonal overlayer. The Fe–K distance, as determined from the measurements in the (2 2 0) Bragg reflection, is 3.4±0.2 Å. The long bridge site seems to be the preferred adsorption site for the potassium atoms in the distorted hexagonal overlayer. This geometry not only fits all the XSW data, but also explains all spots in the LEED pattern without the need to introduce multiple scattering. Comparison of the measured and simulated XSW data, based on the distorted hexagonal overlayer, enables a more accurate determination of the Fe–K bond length to 3.36±0.14 Å. This corresponds to a potassium hard sphere radius of r_K=2.12±0.14 Å. This radius is among the largest reported for potassium on a metal, which is attributed to the high coverage and coordination of the K atoms in this overlayer arrangement.     

The adsorption position of Hg on Ni(100): a transmission channeling study

The adsorption position of Hg on Ni(100) has been studied with transmission channeling. It is shown that for coverages up to 0.5 monolayer (ML), Hg adsorbs in the four-fold hollow site (FFHS) at a height of 2.25 ± 0.10 rA. This is at variance with an earlier report of adsorption in the bridge site. For a crystal temperature of 115 K and low coverage (θ_Hg t 0.18 ML), the two-dimensional vibration amplitude parallel to the surface, ρ_Hg, is 0.10 +- 0.03 rA, but at higher coverage, θ_Hg 0.45 ML, there is an apparent increase in the root-mean-square displacement of the Hg atoms from the FFHS. Possible origins of this increase are discussed.     

The effect of potassium on the molecular orientation of CO chemisorbed on Ni(111): An esdiad study

Using electron stimulated desorption (ESD) and electron stimulated desorption ion angular distribution (ESDIAD) techniques, we have determined that coadsorbed potassium systematically quenches the O⁺ ion yield from CO on the Ni(111) surface for 1000 eV electron excitation energies. The quenching appears to be a short range K-CO interaction; 3 or 4 CO molecules are affected for each K atom adsorbed on the surface. The quenching effect of K on CO indicates that a significant electronic perturbation of CO is caused by its local interaction with K. This effect prevents ESDIAD observation of the K-quenched CO species. In addition, the CO molecules that are not quenched at a potassium coverage of 0.02 K/Ni exhibit a normally oriented C-O bond similar to that found for CO adsorbed on a K-free Ni(111) surface.     

Large solid angle X-ray photoelectron intensity distributions from CO, methoxy, and formate adsorbed on Ni(110)

Adsorbed layers of CO, methoxy (CH₃O) and formate (HCOO) on a clean Ni(110) surface are investigated by angle-resolved X-ray photoelectron spectroscopy. Large solid angle stereographic projections of C 1s or O 1s core level intensity measured for these adsorbates at 85 K show strong forward scattering enhancements that unambiguously and rather directly determine the molecular orientation, and in the case of CO, the order within the layer. CO in the (2×1) p2mg structure and methoxy are tilted by 20° and 30° relative to the normal, respectively, with the projection of the tilt in the 001 azimuth. Formate is bonded through oxygen in the bidendate configuration, aligned along the 110 azimuth. The O–C–O bond angle is determined as 124°. These results are in good agreement with previous data.     

C_2H_x(x=4～6)在Ni(111)表面吸附的DFT研究

采用密度泛函理论与周期平板模型相结合的方法,对物种C_2H_x(x=4～6)在Ni(111)表面的top,fcc,hcp和bridge位的吸附模型进行了结构优化、能量计算,得到了各物种较有利的吸附位;并对最佳吸附位进行密立根电荷和总态密度分析.结果表明:C_2H_6和C_2H_4在Ni(111)表面的最稳定吸附位都是top位,吸附能分别是-36.41和-48.62 kJ·mol~(-1),物种与金属表面吸附较弱;而C_2H_5在Ni(111)表面的最稳定吸附位hcp的吸附能是-100.21 kJ·mol~(-1),物种与金属表面较强;三物种与金属表面之间都有电荷转移,属于化学吸附.     

The structures of sulphur on Pd(111) studied by X-ray standing wavefield absorption and surface EXAFS

The surface structures of R30°-S and R19.1°-S on Pd(111) have been investigated by normal incidence X-ray standing wave (NIXSW) absorption and surface extended X-ray absorption fine structure (SEXAFS). NIXSW measurements show that the most likely site of S adsorption in the R30° phase is the threefold “fcc” hollow. The location of the S atoms at the “fcc” hollow site is consistent with S adsorption on the neighbouring fcc (111) transition metal surfaces. SEXAFS analysis revealed a S–Pd nearest neighbour bond distance of 2.28±0.04 Å. The results for the R19.1° phase suggest that the structure involves a mixed S–Pd overlayer, with the S–Pd vertical layer spacing equal to the Pd bulk 111 spacing.     

Bromine adsorption on Cu(111)

The dissociative chemisorption of molecular bromine on Cu(111) at 300 K has been studied using ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change measurements. A (√3 × √3)R30° structure is formed initially at a bromine coverage of 0.33 ML. This then converts to a (9√3 × 9√3)R30° compression structure with a coverage of 0.41 ML. The coincidence distance of the compression structure is determined entirely by the van der Waals diameter of adsorbed bromine. The applicability of using the van der Waals diameters of the three halogens, Cl, Br and I, to predict the saturation compression structures on Cu(111), is discussed.     

The decomposition of H2S on Ni(110)

Adsorbed H₂S decomposes on Ni(110) to form primarily surface S and H for coverages of less than 0.5 ML. The hydrogen evolves in two separate TPD peaks, characteristic of hydrogen recombination and desorption from the clean surface and from regions perturbed by chemisorbed sulfur. XPS and HREELS indicate the presence of SH and possibly H₂S groups on the surface at 110 K. The XPS data indicates that for coverages less than about 0.5 ML, the concentration of molecular H₂S is small, but it is difficult to asess the coverage of SH groups. However, all of the molecular species decompose prior to hydrogen desorption (for high coverage, 180 K). Physisorbed H₂S is observed on the surface for coverages greater than about 0.5 ML.

The sulfur Auger lineshape was observed to be a function of both coverage and temperature. The changes in the lineshape were attributed to perturbations in local bonding interactions between the S and the Ni surface, perhaps involving some change in either bonding sites or distances but not involving SH bond scission.

The decomposition reaction was modeled using a bond order conservation method which successfully reproduced the experimental results.     

Ni(111)表面C原子吸附的密度泛函研究

基于密度泛函理论的第一性原理计算,对过渡金属Ni晶体与Ni (111)表面的结构和电子性质进行了研究, 并探讨了单个C原子在过渡金属Ni (111)表面的吸附以及两个C原子在Ni(111)表面的共吸附. 能带和态密度计算表明, Ni晶体及Ni (111)表面在费米面处均存在显著的电子自旋极化. 通过比较Ni (111)表面各位点的吸附能,发现单个C原子在该表面最稳定的吸附位置为第二层Ni原子上方所在的六角密排洞位, 吸附的第二个C原子与它形成碳二聚物时最稳定吸附位为第三层Ni原子上方所在的面心立方洞位. 电荷分析表明,共吸附时从每个C原子上各有1.566e电荷转移至相邻的Ni原子, 与单个C原子吸附时C与Ni原子间的电荷转移量(1.68e)相当. 计算发现两个C原子共吸附时在六角密排洞位和面心立方洞位的磁矩分别为0.059μ_B和 0.060μ_B,其值略大于单个C原子吸附时所具有的磁矩(0.017μ_B).     

Local structure of OH adsorbed on the Ge(0 0 1)(2 × 1) surface using scanned-energy mode photoelectron diffraction

The local geometry of OH fragments adsorbed on the Ge(0 0 1)(2 × 1) surface has been examined using O 1s scanned energy mode photoelectron diffraction. These fragments were obtained by the dissociative reaction of the clean surface with H₂O. The Ge–O bond length is found to be 1.76 ± 0.02 Å and the Ge–O bond angle to be 15° ± 2° relative to the surface normal. Some information about the positions of the Ge dimer atoms has also been obtained.     

Local adsorption sites and bondlength changes in Ni(1 0 0)/H/CO and Ni(1 0 0)/CO

The local adsorption geometry of CO adsorbed in different states on Ni(1 0 0) and on Ni(1 0 0) precovered with atomic hydrogen has been determined by C 1s (and O 1s) scanned-energy mode photoelectron diffraction, using the photoelectron binding energy changes to characterise the different states. The results confirm previous spectroscopic assignments of local atop and bridge sites both with and without coadsorbed hydrogen. The measured Ni–C bondlengths for the Ni(1 0 0)/CO states show an increase of 0.16 ± 0.04 Å in going from atop to bridge sites, while comparison with similar results for Ni(1 1 1)/CO for threefold coordinated adsorption sites show a further lengthening of the bond by 0.05 ± 0.04 Å. These changes in the Ni–CO chemisorption bondlength with bond order (for approximately constant adsorption energy) are consistent with the standard Pauling rules. However, comparison of CO adsorbed in the atop geometry with and without coadsorbed hydrogen shows that the coadsorption increases the Ni–C bondlength by only 0.06 ± 0.04 Å, despite the decrease in adsorption energy of a factor of 2 or more. This result is also reproduced by density functional theory slab calculations. The results of both the experiments and the density functional theory calculations show that CO adsorption onto the Ni(1 0 0)/H surface is accompanied by significant structural modification; the low desorption energy may then be attributed to the energy cost of this restructuring rather than weak local bonding.     

The adsorption of carbon monoxide on TiO2(110) supported palladium

Palladium overlayers deposited on TiO₂(110) by metal vapour deposition have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO. Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively in a bridged configuration with a band (B₁ at 1990 cm⁻¹) characteristic of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to 500 K, XPS and LEED indicate the nucleation of Pd particles on which CO adsorbs predominantly as a strongly bound linear species which we associate with edge sites on the Pd particles (L* band at 2085 cm⁻¹). Both bridged and linear CO bands are exhibited as increases in reflectivity at the resonant frequency, indicating the retention of small particle size during the annealing process. Palladium overlayers of intermediate coverages (10–20 ML) deposited at 300 K undergo some nucleation during growth, and adsorbed CO exhibits both absorption and transmission bands in the B₁ (1990 cm⁻¹) and B₂ (1940 cm⁻¹) regions. The latter is associated with the formation of Pd(111) facets. Highly dispersed Pd particles are produced on annealing at 500 K. This is evidenced by the dominance of transmission bands for adsorbed CO and a significant concentration of edge sites, which accommodate the strongly bound linear species at 300 K. Adsorption of CO at low temperature also allows the identification of the constituent faces of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets during the annealing process. High coverages of palladium (100 ML) produce only absorption bands in FT-RAIRS of adsorbed CO associated with the Pd facets, but annealing these surfaces also shows a conversion to Pd(111) facets. LEED indicates that at coverages above 10 ML, the palladium particles exhibit (111) facets parallel to the substrate and aligned with the TiO₂(110) unit cell, and that this ordering in the particles is enhanced by annealing.     

The structure of ammonia molecules on MgO(100) surfaces from monolayer to bulk condensation

The structure of ND₃ molecules adsorbed on MgO (100) surfaces has been studied by neutron diffraction within the 10–80 K temperature range and at 0.7, 1 and 2.3 monolayer coverage. The neutron spectra suggest that the monolayer presents a short range order with a hcp packing of ammonia molecules, a coherence length of 25 ± 2 Å and a nearest neighbour distance of 3.61 ± 0.04 Å The molecular spacing remains the same between 10 and 80K that we interpret as small higher order commensurate islands. Above one monolayer coverage, bulk crystallites form on top of the first monolayer.