Structure and composition of adsorbed layers formed by sequential exposure of Pt(100) and Pt(111) to pairs of compounds: Solvents and electrolytic substances

Studies are reported of the interaction of vapor of typical polar solvents and electrolytes at electrodes having Pt(111) or Pt(100) single-crystal surfaces: water, pyridine, acetonitrile, dimethyl-sulfoxide, hydrogen bromide, iodine, sulfur dioxide, acrylic acid, and ammonia. Exposure was extended from low pressures (about 10^?5 Torr) to pressures approaching the vapor pressure of the pure liquid. The results indicate that these typical electrochemical materials adsorb strongly to the clean Pt surface but once adsorbed tend not to react with each other. However, analysis of LEED patterns and Auger intensities suggests that exposure of an adsorbed layer of solvents such as dimethylsulfoxide to iodine results in adsorption of the halogen and molecular re-orientation of the adsorbed solvent.     

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd_0.75Cu_0.25(111) single crystal surface with surface composition of about Pd_0.7Cu_0.3. The main objective was to make a comparison with the previously studied Pd_0.67Ag_0.33(111) (surface composition Pd_0.1Ag_0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10⁻⁹ mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.     

The role of HH interactions in the formation of ordered structures on Ni and Pd single crystals

The interaction between H adatoms on a surface is calculated within the embedded cluster model of chemisorption. The model is first applied to the case of two H atoms on a free electron surface. The interaction energy is found to be an oscillatory function of the H-H separation R_ab. Application of the free electron model to the problem of chemisorption on transition metal surfaces leads to unphysical results with the prediction of formation of ordered H overlayers which are not observed in LEED experiments. We next include the l = 2 TM muffin tins. Results for H adsorption on the low index faces of Ni and Pd substrates are presented. Graphitic structures are predicted for the (111) faces of both Ni and Pd with the H atoms occupying both types of three-fold hollow sites on the surface. This agrees with the results of LEED experiments for H/Ni(111). Comparison with experiment is not possible in the case of H/Pd(111) owing to the lack of low temperature studies for that system. Zig-zag chains with the H atoms adsorbed in sites of three-fold coordination on alternate sides of the TM(110) rows are predicted for both Ni and Pd. This is in agreement with the results of He diffraction experiments for H/Ni(110). No structure determination has been done for H/Pd(110). Adsorption in the four-fold centre sites for H on the (100) faces of Ni and Pd is found to be unfavourable. The H atoms are expected to adsorb in sites of three-fold symmetry below the (100) surface for H on Pd with formation of a c(2 × 2) structure in agreement with the LEED observations. For H/Ni(100) the H atoms are believed to adsorb above the surface, away from the centre site and to bond to two surface Ni atoms. No short-range ordered structures are predicted in this case.     

Adsorption of hydrogen on palladium single crystal surfaces

The adsorption of hydrogen on clean Pd(110) and Pd(111) surfaces as well as on a Pd(111) surface with regular step arrays was studied by means of LEED, thermal desorption spectroscopy and contact potential measurements. Absorption in the bulk plays an important role but could be separated from the surface processes. With Pd(110) an ordered 1 × 2 structure and with Pd(111) a 1 × 1 structure was formed. Maximum work function increases of 0.36, 0.18 and 0.23 eV were determined with Pd(110), Pd(111) and the stepped surface, respectively, this quantity being influenced only by adsorbed hydrogen under the chosen conditions. The adsorption isotherms derived from contact potential data revealed that at low coverages θ ∞ √p_H2, indicating atomic adsorption. Initial heats of H₂ adsorption of 24.4 kcal/mole for Pd(110) and of 20.8 kcal/mole for Pd(111) were derived, in both cases E_ad being constant up to at least half the saturation coverage. With the stepped surface the adsorption energies coincide with those for Pd(111) at medium coverages, but increase with decreasing coverage by about 3 kcal/mole. D₂ is adsorbed on Pd(110) with an initial adsorption energy of 22.8 kcal/mole.     

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.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

A leed-aes study of thin Pd films on Si(111) and (100) substrates

A system Pd (deposit)-Si (substrate) has been studied by LEED and AES. Pd₂Si formed on Si(111) became epitaxial after a short time of annealing at a temperature between 300 and 700°C, while the Pd₂Si formed on Si(100) did not, in both cases the surfaces of the Pd₂Si being covered with a very thin Si layer. A sequence of superstructures (3√3 × 3√3), (1 × 1), and (2√3 × 2√3) was observed successively in Pd/Si(111) as the annealing temperature was increased. A (√3 × √3) structure was obtained by sputtering the 3√3 surface slightly. It was found that the √3 structure corresponds to Pd²Si(0001)-(1 × 1) grown epitaxially on Si(111), and that the 3√3 structure comes from the thin Si layer accumulated over the silicide surface, while the 2√3 and 1 structures arise from a submonolayer of Pd adsorbed on Si(111). Superstructures observed on a Pd/Si(100) system are also studied.     

AES and LEED studies of xenon adsorption on (100)NaCl

The thermal and electro impact behaviour of NO adsorbed on Pt(111) and Pt(110) have been studied by LEED, Auger spectroscopy, and thermal desorption. NO was found to adsorb non-dissociatively and with very similar low coverage adsorption enthalpies on the two surfaces at 300 K. In both cases, heating the adlayer resulted in partial dissociation and led to the appearance of N₂ and O₂ in the desorption spectra. The (111) surface was found to be significantly more active in inducing the thermal dissociation of NO, and on this surface the molecule was also rapidly desorbed and dissociated under electron impact. Cross sections for these processes were obtained, together with the desorption cross section for atomically bound N formed by dissociation of adsorbed NO. Electron impact effects were found to be much less important on the (110) surface. The results are considered in relation to those already obtained by Ertl et al. for NO adsorption on Ni(111) and Pd(111), and in particular, the unusual desorption kinetics of N₂ production are considered explicitly. Where appropriate, comparisons are made with the behaviour of CO on Pt(111) and Pt(110), and the adsorption kinetics of NO on the (110) surface have been examined.     

The sorption of CO and O on Ni (111) studied by leed and iss

The adsorption of CO and O on Ni (111) was studied by low-energy ion scattering (ISS) and low-energy electron diffraction (LEED). For the ordered (√7/2) × (√7/2) R19.1° CO layer ion scattering gives a coverage greater than $\text{1}\text{2}$ monolayer, and for the (2 × 2) O layer a coverage of $\text{1}\text{4}$ monolayer. The CO is non-dissociatively adsorbed, with the C bound to the Ni. The molecules are oriented parallel to the surface normal. Island formation at lower CO coverages is possible.     

The adsorption of activated nitrogen on platinum single crystal faces

The adsorption of activated nitrogen on a stepped Pt(S)-[9(111) × (111)] face was investigated by LEED, AES and flash desorption. Nitrogen was supplied to the crystal from a high frequency discharge tube. For comparison some orienting measurements were also carried out on smooth (111) and (100) platinum faces. Activated nitrogen is adsorbed at room temperature on all three faces up to about half a monolayer coverage. No additional LEED patterns indicating long range order of the adsorbed layer were found. By flash heating a small desorption peak at 120°C and a large peak between 175 and 230°C depending on the initial coverage were observed on the (111) type faces. The desorption can be described approximately by a second order rate law with an energy of activation of 25± 3 kcal/mole. No influence of surface steps on the properties of the adsorbed layer was detected. On the (100) face two coverage independent desorption maxima at 120 and 170°C of about equal intensities were found.     

Surface coverage measurements by sims for CO adsorption on a number of metals and for CO and H2S CO-adsorption on Ni(110), (100) and (111)

A detailed study of CO adsorption on Ni(100) utilizing static SIMS and a comparison of the data with surface coverage data from the literature shows that there is a linear relationship between CO coverage and the peak intensity ratios (MCO⁺/M⁺ and M₂CO⁺/M⁺₂) of the CO-containing secondary ions, in the region of coverage below which the adlayer becomes compressed. Adsorption isobares were obtained using the intensity ratios and from these, adsorption isosteres were derjved to give heats of adsorption as a function of coverage. These data are in very close agreement with the literature. Confirmatory data were obtained for this relationship for CO adsorption on polycrystalline Ni, Pd, Pt and Cu and Cu(100). The application of this technique of surface coverage measurements to a study of the extent to which H₂S coadsorption reduces the coverage of adsorbed CO on Ni(110), (100) and (111) shows that these faces are poisoned in the order (100) > (111) > (110). Surface coverage measurements on the non-closepacked (110) face are affected by the apparent insensitivity of SIMS to adsorbates located in the “channels”.     

Ni(111)表面一氧化碳和氢共吸附的密度泛函理论研究

本文用密度泛函理论(DFT)的总能计算研究了一氧化碳和氢原子在Ni(111)表面上p(2×2)共吸附系统的原子结构和电子态，结果表明CO和H原子分别被吸附于两个对角p(1×1)元胞的hcp和fcc位置.以氢分子和CO分子作为能量参考点，总吸附能为2.81 eV，相应的共吸附表面功函数φ为6.28 eV.计算得到的C—O，C—Ni和H—Ni的键长分别是1.19?, 1.96?和 1.71?，并且CO分子以C原子处于hcp的谷位与金属衬底原子结合.衬底Ni(111)的最外两层的晶面间距在吸附后的相对变化分别是 关键词： Fisher-Tropsch反应 催化作用 Ni(111) p(2×2)/(CO+H) 共吸附     

Vibrational EELS studies of CO chemisorption on clean and carbided (111), (100) and (110) nickel surfaces

Room temperature adsorption of CO on bare and carbided (111), (100) and (110) nickel surfaces has been studied by vibrational electron energy loss spectroscopy (EELS) and thermal desorption. On the clean (100) and (110) surfaces two configurations of CO adsorbed species, namely “terminal” and bridge bonded CO, are observed simultaneously. On Ni(111), only two-fold sites are involved. The presence of superficial carbon lowers markedly the bond strength of CO on Ni(111)C and Ni(110)C surfaces, while no adsorption has been detected on the Ni(100)C surface. Moreover, on the carbided Ni(110)C surface, the adsorption mode for adsorbed CO is changed with respect to the clean surface; only “terminal” CO is then observed.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

Leed theory for incommensurate overlayers: Application to graphite on Pt(111)

A dynamical LEED analysis is applied to study the structure of incommensurate overlayers. The previously described BSN method is extended to the incommensurate case, providing an efficient theoretical scheme for calculating the required LEED beam intensities. This method is used to investigate the structure of an incommensurate graphite overlayer on the Pt(111) surface by analyzing the specular diffraction beam intensities at various angles. The measured LEED intensities are well represented by a surface model consisting of a graphite layer 3.70 ± 0.05 Å above the Pt(111) surface, supported by at least a partial layer of “intercalated” carbidic carbon atoms chemisorbed in three-fold hollow sites 1.25 ± 0.10 Å above the Pt(111) surface. A “data subdivision method” is applied in the R-factor analysis to distinguish between the different R-factor minima.     

The adsorption of no on Ni(111); an esdiad/leed study

The ESDIAD method (electron stimulated desorption ion angular distributions) has been combined with LEED (low energy electron diffraction) in a study of the adsorption of NO on Ni(111). For adsorption at 80 K, NO appears to be bonded with its molecular axis perpendicular to the Ni(111) surface at all coverages. Heating the 80 K layer leads to a striking structural change which we interpret as the formation of inclined or bent NO in the range 120 ? T ? 250 K. Upon adsorption at 150 K, only the bent form of NO is present at low coverages; at higher coverages at 150 K, the perpendicular form appears, in agreement with recent electron energy loss spectroscopy (EELS) data of Lehwald, Yates, and Ibach. When NO is coadsorbed with p(2 × 2) oxygen, the perpendicular form of NO dominates at all coverages and temperatures studied. Dissociated NO adsorbed at steps and defect sites on Ni(111) produces a welldefined hexagonal ESDIAD pattern.     

Adsorption of CO on Pd single crystal surfaces

Studies of CO adsorption on Pd(110), (210) and (311) surfaces as well as with a (111) plane with periodic step arrays were performed by means of LEED, contact potential and flash desorption measurements. Isosteric heats of adsorption were evaluated from adsorption isotherms. Earlier work with Pd(111) and Pd (100) surfaces is briefly reviewed, yielding the following general picture: The initial adsorption energies vary between 34 and $\text{40}\text{kcal}\text{mole}$ and close similarities exist for the dipole moments, the maximum densities of adsorbed particles and for the adsorption kinetics. At low and medium coverage the adsorbed particles are located at highly symmetrical adsorption sites, whereas saturation is characterized by the tendency for formation of close-packed layers.     

Adsorption of carbon monoxide on silver/palladium alloys

The adsorption of carbon monoxide on epitaxial (100) and (111) planes of Ag/Pd alloys with definite surface compositions has been studied by means of LEED, Auger electron spectroscopy and work function measurements. The formation of ordered adsorbed structures is prevented by even small amounts of silver in the surfaces. The maximum variation of the work function with CO adsorption bears no simple relationship to the surface composition. From measured adsorption isotherms the isosteric heats of adsorption have been evaluated. For CO adsorption on pure Pd planes the adsorption energies E_ad are either constant or decrease slowly up to high coverages, whereas a continuous decrease was observed with the alloys indicating the energetical heterogeneity. The results are discussed on the basis of our knowledge about the nature of the CO chemisorption and about the electronic structure of Ag/Pd alloys.     

Substitutional adsorption of K on Fe(100)

The adsorption of potassium on Fe(100) was studied by time-of-flight forward scattering and recoiling spectroscopy (TOF-SARS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). After heating to 650 K of the potassium saturated surface the formation of a p(3 × 3) potassium superstructure was observed by LEED. TOF-SARS experiments ruled out the adsorption of potassium in the on-top, bridge and four-fold hollow site. The only site which is in agreement with all experimental results is the substitutional site where K replaces an Fe atom of the topmost layer of the crystal. This is the first time a substitutional adsorption site has been found on a bcc surface. On an fcc surface such an adsorption site has been found recently for adsorption of sodium and potassium on Al(111).     

Order-disorder transitions in a chemisorbed system: Hydrogen on Ni (111)

Atomic hydrogen chemisorbed on a Ni (111) surface forms at coverages between 0.3 and 0.6 an ordered 2 × 1-structure as observed by low energy electron diffraction (LEED). The intensity of the fractional-order LEED spots was measured at different coverages as a function of temperature. Continuous order-disorder transitions are found, the maximum transition temperature (270 K) being at θ = 0.5. The phase diagram, however, is asymmetric with respect to this coverage and can therefore presumably not be explained on the basis of (independent) pairwise interactions.