Reconstruction behaviour of fcc(110) transition metal surfaces and their vicinals

The fcc(110) surfaces are well known for their strong tendency to missing-row (MR) type reconstructions either in the clean state (Au, Pt) or driven by adsorbates (Ni, Cu, Pd, Ag). The present knowledge on the different reconstruction behaviour of flat (110) surfaces is reviewed. The survey focuses on recent scanning tunneling microscopy (STM) studies, which for the first time also elucidate the dynamics of the reconstruction process for the various systems. An overview of our recent STM and low energy electron diffraction studies on vicinal Au(110) and Ni(110) surfaces is given, aiming for a deeper understanding of the influence of steps on reconstruction behaviour of fcc(110) surfaces on the one hand, and on the stability of reconstructing vicinal surfaces on the other. Finally, we report on the reconstruction behaviour of Ir(110), which stabilizes in the clean state by formation of mesoscopic (331) facets and dereconstructs to the (1×1) phase upon oxygen adsorption at 700–900 K.     

Field-induced transfer of lithium in a scanning tunneling microscope

Field-induced transfer of Li atoms from a [110]-oriented W tip to a Pt sample is realized in the scanning tunneling microscope (STM) under UHV conditions, using a working principle of the new solid state Li surface diffusion metal ion source (Li-SDMIS), in which the supply of Li to the apex of the tip occurs via surface diffusion. By applying 5–10 μs voltage pulses to the Li-covered W tip placed within tunneling range, single Li hillocks (400 × 400 Ų) were formed on the chosen area of the Pt(110) sample in a well-rep way. The pulse magnitude necessary for Li emission displays a distinct threshold character. A comparative analysis of the energetics of Li field desorption in the STM and Li-FDM (lithium field desorption microscope) modes indicates chemically-assisted field desorption of Li as the field-induced transfer mechanism.     

NO adsorption on the Rh(110) surface: kinetics and composition of the adlayer studied by fast XPS

The adsorption and dissociation of NO on the Rh(110) surface were studied by synchrotron radiation X-ray photoemission spectroscopy at temperatures in the range 210–370 K. The O 1s or N 1s spectra were collected every 14 s while the surface was continuously exposed to a steady NO gas pressure. The difference in the binding energies for the atomic oxygen (O 1s ≤530.2 eV), atomic nitrogen (N 1s 397.2 eV) and molecular upright bonded NO molecules (O 1s ≥531.0 eV and N 1s 400 eV) allowed us to distinguish these surface species and to follow the evolution of the adsorbate layer. In addition to these dominating surface species a new species, characterized by O 1s binding energy of 530.7 eV and N 1s binding energy similar to that of the atomic nitrogen, was detected within a narrow coverage range. This state is tentatively assigned to a “lying down” NO bonding configuration, detectable at the timescale of the measurements. The uptake plots, constructed using the integrated intensity of the deconvoluted O 1s and N 1s spectra, are used to elucidate the effect of the reaction temperature and surface coverage and composition on the kinetics of dissociative and molecular NO adsorption of Rh(110).     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Adsorption and reactions of CH2I2 on Ru(001) surface

The adsorption and dissociation of CH₂I₂ were studied at 110 K with the aim of generating CH₂ species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH₂I₂ is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH₂I₂ has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer (T_p=200 K), a weakly bonded state (T_p=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH₃I, which desorbs with T_p=160 K. The adsorption of CH₂I₂ at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH₂ and I proceeded at 198–230 K, as indicated by a shift in the I(3d_5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH₂ is self-hydrogenated into CH₄ (T_p=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH₂I₂ initiated the dissociation of CH₂I₂ monolayer even at 110 K, and affected the reaction pathways of CH₂.     

Growth and thermal properties of ultrathin cerium oxide layers on Rh(1 1 1)

Ultrathin layers of cerium oxide have been deposited on a Rh(1 1 1) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO₂ islands at elevated substrate temperature (250–300 °C), with (1 1 1) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce³⁺ species, which appear to be located predominantly at the ceria–Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO₂ is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 °C, the ceria decomposes and Ce–Rh alloy phases are identified.     

Characterization of doped diamond-like carbon films deposited by hot wire plasma sputtering of graphite

Diamond-like carbon (DLC) films doped with nitrogen and oxygen were deposited on silicon(100) and polytetrafluoroethylene (PTFE) substrates by hot wire plasma sputtering of graphite. The morphology and chemical composition of deposited films has been characterized by scanning electron microscopy, XPS, Auger, FTIR spectroscopy and micro-Raman scattering. Plasmon loss structure accompanying the XPS C 1s peak and electron energy loss spectroscopy (EELS) in reflection mode was used to study the fraction of sp³ bonded C atoms and the density of valence electrons. Raman spectra show two basic C–C bands around 1575 cm^-1 (G line) and 1360 cm^-1 (D line) . Auger depth profiling spectroscopy was used to measure the spatial distributions of C, N and O atoms in the surface layer of DLC films. The fraction of sp³ bonded atoms of about 40% was detected in DLC films by XPS plasmon loss and EELS techniques. Nitrile and iso-nitrile groups observed in FTIR spectra demonstrated the existence of sp bonded carbon in doped DLC films. The typical for DLC films specific density 1.7–1.8 g/cm³ was obtained from EELS and XPS data. PACS 52.77.Dq; 81.65.-b; 82.80.Pv     

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH_x(a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.     

The adsorption of NO on an oxygen pre-covered Pt(1 1 1) surface: in situ high-resolution XPS combined with molecular beam studies

Adsorption of NO on a Pt(1 1 1) surface pre-covered with a p(2 × 2) atomic oxygen layer has been studied in situ by high-resolution X-ray photoelectron spectroscopy and temperature-programmed XPS using third-generation synchrotron radiation at BESSY II, Berlin, combined with molecular beam techniques and ex situ by low energy electron diffraction and temperature-programmed desorption. O 1s XP spectra reveal that an ordered p(2 × 2)-O layer dramatically changes the adsorption behavior of NO as compared to the clean surface. The atomic oxygen occupies fcc hollow sites, and therefore blocks NO adsorption on these sites, which are energetically preferred on clean Pt(1 1 1). As a consequence, NO populates on-top sites at low coverage. At 110 K for higher coverages, NO can additionally adsorb on hcp hollow sites, thereby inducing a shift of the O 1s binding energy of atomic oxygen towards lower energies by about 0.25 eV. The bond strength of the hcp hollow NO species to the substrate is weakened by the presence of atomic oxygen. A sharp p(2 × 2) LEED pattern is observed for NO adsorption on the oxygen pre-covered surface, up to saturation coverage. The total saturation coverage of NO on Pt(1 1 1) pre-covered with varying amounts of oxygen (below 0.25 ML) decreases linearly with the coverage of oxygen. The initial sticking coefficient of NO is reduced from 0.96 on clean Pt(1 1 1) to 0.88 on a p(2 × 2) oxygen pre-covered surface.     

Vacancy-assisted diffusion of alkoxy species on rutile TiO2(110)

Using scanning tunneling microscopy (STM) and density functional theory simulations, we have studied the diffusion of alkoxy species formed by the dissociation of alcohols on bridge-bonded oxygen (BBO) vacancies (BBO(V)'s) on TiO2(110). At elevated temperatures (>or=400 K) the sequential isothermal STM images show that mobile BBO(V)'s mediate the diffusion of alkoxy species by providing space for alkyl-group-bearing BBO atom to diffuse into. The experimental findings are further supported by simulations that find that BBO(V) diffusion is the rate limiting step in the overall diffusion mechanism.     

Growth mechanism of the Pd(100)-p(2×2)-p4g-Al surface alloy

We have studied the growth mechanism of a Pd(100)-p(2×2)-p4g-Al surface alloy by scanning tunneling microscopy (STM). The surface alloy has a bilayer structure and is formed by annealing at 450–700 K (depending on the initial aluminum coverage) after the deposition of aluminum on Pd(100) at room temperature. The ratio of the surface-alloy coverage to the initial aluminum coverage is found to be constant (0.44) irrespective of the initial aluminum coverage from 0.5 monolayers (ML) up to 2 ML. The growth mechanism of the surface alloy is proposed on the basis of the STM measurements at various annealing temperatures. Upon annealing at 450 K, some of the surface aluminum atoms migrate into the bulk and, instead, palladium atoms come out to the surface. These palladium atoms react with aluminum atoms remaining on the surface to form a surface alloy. When the initial aluminum coverage is less than 1 ML, bilayer-high islands of the surface alloy with an average area of 100 nm² are formed at 450–500 K, which diffuse on the terrace at 500–700 K and coalesce to form larger islands. A possible role of the percolation transition of aluminum islands in the formation of the surface alloy is discussed.     

X-Ray photoelectron study of the reaction of oxygen with cerium

A clean, polycrystalline Ce-metal surface has been successfully produced by using a sputtering and annealing technique on Ce foil. The chemical species conditions at 300 and 120 K have been characterized by XPS. Dissociative chemisorption and reaction occurs to produce a thick layer of Ce₂O₃. N over this Ce₂O₃. Two peaks occur in the O(1s) spectra, at BE 529.6 and 530.3 eV, which are assigned to CeO₂ and Ce₂O₃, respecti eV becomes clearly apparent when CE(IV) forms. The principal species formed are oxides, and no positive evidence was found for the existence of physiso     

TPD, HREELS and UPS study of the adsorption and reaction of methyl nitrite (CH3ONO) on Pt(111)

The adsorption and reaction of methyl nitrite (CH₃ONO, CD₃ONO) on Pt(111) was studied using HREELS, UPS, TPD, AES, and LEED. Adsorption of methyl nitrite on Pt(111) at 105 K forms a chemisorbed monolayer with a coverage of 0.25 ML, a physisorbed second layer with the same coverage that desorbs at 134 K, and a condensed multilayer that desorbs at 117 K. The Pt(111) surface is very reactive towards chemisorbed methyl nitrite; adsorption in the monolayer is completely irreversible. CH₃ONO dissociates to form NO and an intermediate which subsequently decomposes to yield CO and H₂ at low coverages and methanol for CH₃ONO coverages above one-half monolayer. We propose that a methoxy intermediate is formed. At least some C–O bond breaking occurs during decomposition to leave carbon on the surface after TPD. UPS and HREELS show that some methyl nitrite decomposition occurs below 110 K and all of the methyl nitrite in the monolayer is decomposed by 165 K. Intermediates from methyl nitrite decomposition are also relatively unstable on the Pt(111) surface since coadsorbed NO, CO and H are formed below 225 K.     

Oxygen adsorption on Pt(110)-(1×2): new high-coverage structures

From an interplay between scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, a comprehensive picture is obtained for oxygen adsorption on the Pt(110)-(1×2) surface, from single isolated oxygen atoms chemisorbed in FCC sites along the platinum ridges to the formation of a new high-coverage oxide-like structure with a local coverage of two oxygen atoms per platinum surface atom. We find that the repulsive O–O interactions for the O/Pt(110) system are compensated by an effective O–O attractive interaction originating from a strong coupling between oxygen adsorption and platinum lattice distortions.     

Direct reaction of adsorbed molecular oxygen with hydrazine on a clean Pt(111) surface

The oxidation of hydrazine on the clean Pt(111) surface has been investigated by temperature-programmed reaction spectroscopy (TPRS) in the temperature range 130–800 K. Direct reaction of molecular oxygen is observed on the Pt(111) surface for the first time, as indicated by the desorption of nitrogen beginning at 130 K with a maximum rate at 145 K, below the molecular oxygen dissociation temperature. Direct reaction of hydrazine with adsorbed molecular oxygen results in the formation of water and nitrogen. With excess hydrazine, all surface oxygen is reacted, forming water. When only adsorbed atomic oxygen is present, the low-temperature nitrogen yield decreases by a factor of 3 and the peak nitrogen desorption temperature increases to 170 K. No high-temperature (450–650 K) nitrogen desorption characteristic of nitrogen atom recombination is seen, indicating that during oxidation the nitrogen-nitrogen bond in hydrazine remains intact, as observed previously for hydrazine decomposition on the Pt(111) surface and hydrazine oxidation on rhodium. Two water desorption peaks are observed, characteristic of desorption-limited (175 K) and reaction-limited (200 K) water evolution from the Pt(111) surface. For low coverages of hydrazine, only the reaction-limited water desorption is observed, previously attributed to water formed from adsorbed hydroxyl groups. When excess hydrazine is adsorbed, the usual hydrazine decomposition products, H₂, N₂ and NH₃, are also observed. No nitrogen oxide species (NO, NO₂ and N₂O) were observed in these experiments, even when excess oxygen was available on the surface.     

Adsorption of helical aromatic molecules: heptahelicene on Ni(1 1 1)

The adsorption of the helically shaped polyaromatic hydrocarbon heptahelicene (C₃₀H₁₈) on Ni(1 1 1) was studied by means of STM, TPD, LEED, ToF–SIMS, XPS, and AES at temperatures between 130 and 1000 K. The molecule in the monolayer remains intact up to 500 K. Above that temperature a stepwise decomposition into carbon and hydrogen occurs; the latter desorbing subsequently as H₂. At submonolayer coverages the molecules are adsorbed randomly. The saturated monolayer shows long-range order, but no differences between the structures generated by a pure enantiomer or the racemic mixture have been observed.     

Defect induced surface chemistry: A comparison of the adsorption and thermal decomposition of C2H4 on Rh{111} and Rh{331}

The adsorption and thermal decomposition of C₂H₂ on Rh{111} is compared to the atomically stepped Rh{331} surface over a temperature range of 300 to 800 K. Using X-ray photoelectron spectroscopy (XPS) we find that the C 1s spectra as a function of C₂H₄ exposure exhibit a shift in binding energy (E_b) from 283.5 eV at 1 L C₂H₄ exposure on both surfaces to 283.8 eV on Rh{33 and to 284.1 eV on Rh{111} at saturation coverage (4 L). Careful analysis of the C 1s E_b value and full width at half maximum as a function of surface temperature after a 10 L exposure of C₂H₄ at 300 K reveals that a species consistent with a C₂H adsorbate composition is formed between 400 and 450 K on Rh{111}. This species is also observed on Rh{331} although at the lower temperature of 375 K. Computer peak deconvolution of the C 1s spectra between 500 and 700 K suggests that a CH_ads or C_ads surface fragment is formed and increases in concentration at the expense of the C₂H species as the surface temperature increases. Above 750 K a graphite overlayer is formed on both surfaces. This overlayer, however, exhibits a low degree of carbon π-character bonding on Rh{331}. The adsorption and decomposition mechanisms suggest that the 300 K C₂H₄ adsorbate on Rh{331} is ethylidyne and that the stepped surface is more thermally reactive than the flat Rh{111} surface.     

The adsorption of ethylene oxide on Ag(110) and Pt(111): Relevance to selective olefin oxidation

The interactions of ethylene oxide (EtO) with the Ag(110) and Pt(111) surfaces have been studied using XPS, TDS, AES and EELS. On Ag(110), the interaction is very weak, with only molecular desorption observable. The heat of adsorption is ≈ 10.1 kcal mole⁻¹. In contrast, decomposition reactions strongly predominate on Pt(111) at low coverage. Molecular desorption is only seen at high coverages. The heat of adsorption decreases from > 11.9 to 10 kcal mole⁻¹ with increasing coverage. Condensed multilayers desorb at ≈ 140 K. Ultimate decomposition products on Pt(111) include H₂ and CO gas, and carbon residue on the surface. Evidence suggests that adsorbed decomposition intermediates may include atomic hydrogen, CO, acetyl and ethylidyne species, with at least one other, yet unidentified, species. These results imply that, if produced, adsorbed ethylene oxide would be unlikely to escape a reactor containing Pt catalyst without further decomposition reactions. This may help explain the uniqueness of Ag catalysts in ethylene epoxidation.     

Propylene deep oxidation on the Pt(111) surface: temperature programmed studies over extended coverage ranges

Propylene oxidation was studied on the Pt(111) surface over a wide range of reaction stoichiometries using temperature programmed methods. Reaction of propylene with excess oxygen results in complete oxidation to water and carbon dioxide, with oxydehydrogenation to form water beginning at 290 K. The initiation of skeletal oxidation occurs after water formation begins, except for the highest propylene coverages. A stable dehydrogenated intermediate with a C₃H₅ stoichiometry is formed in the 300 K temperature range during oxidation. Reaction of propylene with substoichiometric amounts of oxygen results in incomplete oxidation with hydrocarbon decomposition dominating after depletion of surface oxygen. Increasing oxygen coverage results in more complete oxidation. Oxidation processes result in water, carbon dioxide, and carbon monoxide, while decomposition results in hydrogen, propylene, and propane desorption with some surface carbon remaining. Propylene-d₆ and selectively labeled propylene-3,3,3-d₃ (CH₂CHCD₃) experiments indicated initial water formation results from oxydehydrogenation of one of the olefinic hydrogens. At the highest propylene and oxygen coverages studied, we observed small amounts of partial oxidation which indicate that the vinyl hydrogen is removed initially, resulting in the formation of an adsorbed H₂CCCH₃ intermediate. The partial oxidation products observed are acetone desorbing at 200 K and acetic acid at 320 K. Removal of the first skeletal carbon begins at 320 K, except for the highest propylene coverages. Preadsorption of molecular oxygen limits adsorption of propylene and preadsorption of propylene limits molecular oxygen adsorption at 110 K. Similar oxidation mechanisms are observed following initial adsorption of both molecular and atomic oxygen, which is expected since molecular oxygen dissociates and/or desorbs well below oxidation temperatures.     

A first-principles study of surface and subsurface H on and in Ni(1 1 1): diffusional properties and coverage-dependent behavior

Periodic, self-consistent, density functional theory (GGA-PW91) calculations are performed for both surface and subsurface atomic hydrogen on and in Ni(1 1 1). At a low coverage (θ=0.25 ML), the binding energies (BEs) of a hydrogen atom in surface fcc, subsurface octahedral (first layer), and subsurface octahedral (second layer) sites are −2.89, −2.18, and −2.11 eV, respectively. The activation energy barriers for hydrogen diffusion from the surface to the first subsurface layer and from the first to the second subsurface layer are estimated to be 0.88 and 0.52 eV, respectively. In the entire coverage range studied, hydrogen occupies surface fcc and hcp sites and subsurface octahedral sites. In addition, the magnitude of the BE per hydrogen atom and the magnetization of the nickel slabs both decrease as hydrogen coverage increases. Vibrational frequencies of hydrogen at various surface and subsurface sites are calculated and are in reasonable agreement with experimental data. A phase stability calculation with a 2 × 2 surface unit cell shows that a p(2 × 2)-2H overlayer structure (θ=0.5 ML) and a p(1 × 1)-1H structure (θ=1.0 ML) are stable at low hydrogen pressures, in agreement with numerous experimental results. A very large increase in pressure is required to populate subsurface sites. After such an increase occurs, the first subsurface layer is filled completely.