Chemisorption of CO on Ir(100) studied by photoemission

Chemisorption of cabon monoxide on a reconstructed Ir(100) surface has been studied by means of LEED and photoemission. In the photoemission spectrum, apart from a low lying peak due to the 4σ-orbital of CO, there is also a broad bump divided by a clear dip. The upper part of this double peak is assigned to the 5σ orbital and the lower part to the 1 π orbital of CO.     

The oxidation of CO on the Pt(100)-(5 × 20) surface

The kinetics of the CO oxidation reaction were examined on the Pt(100)-(5 × 20) surface under UHV conditions. The transient isothermal rate of CO₂ production was examined both for exposure of an oxygen-dosed surface to a beam of CO and for exposure of a CO-dosed surface to a beam of O₂. Langmuir-Hinshelwood kinetics were found to apply in both cases. For the reaction of CO with preadsorbed oxygen atoms, the reaction rate was dependent upon the square-root of the oxygen atom coverage, suggesting that oxygen atoms were adsorbed in islands on this surface. The oxidation of preadsorbed CO was observed only when the initial CO concentrations were less than 0.5 monolayer (c(2 × 2) structure), suggesting that the dissociative adsorption of oxygen required adjacent four-fold surface sites. The activation energy calculated for the reaction of CO with preadsorbed oxygen was 31.4 kcal/mol. This value was 30 kcal/mol greater than the activation energy measured for the reaction of O₂ with preadsorbed CO. Strong attractive interactions within the oxygen islands were at least partially responsible for this difference. The reaction kinetics in both cases changed dramatically below 300 K; this change is believed to be due to phase separation at the lower temperature.     

Non-equilibrium surface phase transitions during the catalytic oxidation of CO on Pt(100)

Isothermal low-pressure oscillations of the rate of catalytic CO oxidation on a Pt(100) surface could be established under appropriate conditions and were monitored through the accompanying periodic variation of the work function. Parallel observations by the Video-LEED technique demonstrated that these oscillations are associated with periodic transformations of the (long-range) surface structure from the reconstructed hex to the 1 × 1 phase and back, which is caused by varying surface concentrations of the reacting particles.     

Mechanism of an adsorbate-induced surface phase transformation: CO on Pt(100)

The mechanism of the (5 × 20)→(1 × 1) transition of Pt(100) during adsorption of CO has been investigated using a fast Video-LEED technique. Analysis of the coverage-dependence of the intensities of several LEED spots on different surfaces leads to a straightforward model. The phase transition occurs by a nucleation-trapping mechanism of the adsorbed CO. The driving force is the difference in stabilities of CO adsorbed on the reconstructed and unreconstructed Pt surfaces.     

Low energy electron spectroscopy of Pt (100) and Pt (100)/CO

Fine structure in the n_vi, _VIIVV spectrum of clean Pt (100) has been observed, and interpreted as “band like” in origin rather than quasi-atomic. Differences in the dependence of the Auger yield on primary beam energy are observed between the N_VI, _VIIVV and O_IIIVV peaks, and are associated with anomalies in the dependence of the inner shell ionization crossection of the 4f level. Low energy electron loss spectra on the clean surface have been investigated at primary energies in the range 71–774 eV and at angles of incidence of the beam 0–60°. The results are related to high energy loss and optical data, and assignments are given for inter-band and plasmon losses. With approximately $\text{3}\text{4}$ of a monolayer of CO on the surface there is a prominent additional loss at around 13.5 eV, which is interpreted as a one electron transition from a σ state below the d band to available states several electron volts above the Fermi level.     

Adsorption of ethylene on the Pt(100) surface

Clean Pt(100) surfaces with bulk-like 1×1 structure, or the stable, reconstructed 5×20 structure and held at 200 or 330 K were exposed to ethylene. Ultraviolet photoemission spectroscopy identified the nature of the adsorbed species which depends on the structure and temperature of the clean surface and the amount adsorbed. It is ethylene on the 5 × 20 structure at 200 K, a vinyl radical on the same surface at 300 K up to half a monolayer, the remainder being added as acetylene; it is acetylene on the 1 × 1 surface at 330 K and a mixture of acetylene, vinyl and ethylene on the 1 × 1 surface at 200 K. Whatever the nature of the adsorbate, the surface coverage θ increased with exposure ? as $\text{(1 ? θ = C?}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>3</mtext>}}\text{)}$. By contrast, on a surface covered with any C₂ hydrocarbon acetylene adsorbs with Langmuir kinetics. The kinetics are explained in terms of the relationship between the attraction an approaching molecule experiences from the bare surface and its Van der Waals repulsion from preadsorbed molecules.     

Calculation of the structural energy of the unreconstructed and (1 × 2) reconstructed Pt(110) surface

Describing the binding energy of both d and s valence electrons within the LCAO formalism, and by including repulsive Born-Mayer type interactions, we study the structural stability of the reconstructed and unreconstructed Pt(110) surfaces. Our main result is that amongst the various models for the (1 × 2) reconstruction the “Bonzel-Ferrer” model is unfavoured, while the “missing-row” model seems to be energetically degenerate with the unreconstructed surface. Our calculation predicts also a small surface concentration, which, however, has only a minor effect on the total energy of the system.     

The structure of the Pt(100) surface

The atomic arrangement in the first two layers of the clean Pt(100) surface is described from a Fourier analysis of low energy electron diffraction (LEED) intensities. The Fourier transform and sensible platinum-platinum bond lengths require that the top layer is corrugated, the corrugation “step” being approximately 0.4 Å. The structure of the first layer is partly described by a hcp rearrangement from the bulk fcc structure. The rearrangement is related to geometrical and electronic properties of the metals. Some qualitative and quantitative comments are made on the general value of Fourier methods in surface crystallography.     

Chemisorption of CO on Cu(100), Ag(110) and Au(110)

The adsorption of CO on Cu, Ag and Au is studied using core and valence photoemission, X-ray absorption and autoionization of core excited states. The purpose is to investigate the nature of the adsorption bond starting out from the well-established chemisorption system CO/Cu(100)-c(2 × 2), and from the results we suggest that CO forms chemisorbed phases also on Ag(110) and Au(110). The photoemission spectra show strong shake-up satellites both for the valence levels and the core levels. The separation to the satellite appearing closest to the main line is observed to follow the position of the substrate d-band relative to the Fermi level. The CO adsorption strength for the noble metals is deduced to decrease in the order Cu-Au-Ag. This is based on the widths of the XA resonances, which are related to the adsorbate-substrate interaction strength of the core excited states, and the relative shake-up intensities, which are expected to increase with a decreasing adsorption strength in the ground state. The same trends regarding the shake-up intensities are observed both for the valence and core levels.     

The NO + CO reaction on Pt(100)

The interaction of NO with CO and with H₂ on Pt(100) was studied by temperature programmed desorption (TPD), isothermal desorption mass spectrometry, and low energy electron diffraction (LEED), TPD of NO and CO coadsorbed at 120 K yields almost complete reaction with both N₂ and CO₂ products desorbing as sharp, simultaneous peaks at ≈ 410 K. with full widths at half maximum as narrow as 3 K. Isothermal desorption mass spectrometry yields N₂ and CO₂ rates that exhibit a maximum with time. Both experiments indicate that the reaction mechanism is autocatalytic. Annealing NO-CO adlayers formed at 120 K to temperatures above 300 K causes the subsequent N₂ and CO₂ TPD peaks to broaden.'TPD of NO coadsorbed with H₂ yields sharp N₂ and H₂O product peaks that closely resemble the N₂ and CO₂ peaks observed in the NO + CO reaction. LEED experiments during TPD and isothermal desorption showed that the (1 × 1) → hex substrate phase transformation sometimes accompanies desorption of N₂ and CO₂. The TPD and isothermal desorption results can be fit by two simple models: chemical autocatalysis, in which an intermediate chemical species participates in a “chain propagation” reaction, and structural autocatalysis, which involves the formation of a reactive intermediate structure involving Pt atom displacements.     

Field ion microscope observations of surface reconstructions on Pt(100) and Ir(100)

The reconstruction of the Pt(100) and Ir(100) plane has been studied by field ion microscopy. Small clusters of atoms produced by low-temperature field evaporation transform to new structures at temperatures above 270 K for Pt and 470 K for Ir. The atomic structure of reconstructed images is not resolved, but the shapes of the clusters suggest reconstructions consistent with current models based on low energy electron diffraction studies. Field evaporation of the reconstructed surfaces indicates that the restructuring extends to the second layer of atoms.     

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.     

Chemisorption of oxygen on the silver (110) surface

In the present work the interaction between oxygen and the silver (110) surface is investigated, mainly using LEED-Auger techniques and thermal desorption spectra. The formation and stability of adsorption layers is studied after exposures at pressures from 10^?3 to 1 torr. At maximum coverage, a (2 × 1) superstructure is formed which is stable up to the desorption temperature. At lower coverage, (3 × 1) and (4 × 1) superstructures are also observed. On the basis of the experimental evidence, tentative models for these structures are presented and discussed.     

On the nonlinear dynamics in the oxidation of CO on Pt(100) and Pt(110) surfaces: Forced oscillations

The effects of low-amplitude periodic modulation of oxygen pressure on the catalytic oxidation on Pt(100) and Pt(110) are investigated with the aid of a set of kinetic equations derived by Cox, Ertl and Imbihl. In the response of the system, subharmonic and superharmonic entrainment bands are found in addition to the fundamental entrainment band. Furthermore, quasiperiodic oscillations are obtained between the entrainment bands.     

Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

A multitechnique surface science examination of Sn deposition on Pt(100)

Interfaces prepared by vapor deposition of Sn onto Pt(100) surfaces have been examined using the following techniques: Auger electron and X-ray photoelectron spectroscopy (AES and XPS), low-energy electron diffraction (LEED), and low-energy ion surface scattering (LEISS) with Ne⁺ ions. Tin deposition was conducted at 320 and 600 K, and the surface composition and order was examined as a function of further annealing to 1200 K. The AES uptake plots (signal versus deposition time) indicate that the Sn growth mode can be described by a layer-by-layer process only up to one adayer at 320 K. Some evidence of 3D growth is inferred from LEED and LEISS data for higher Sn coverages. For deposition at 600 K, AES data indicate significant interdiffusion and surface alloy formation. LEED observations (recorded at a substrate temperature of 320 K) show that the characteristic hexagonal Pt(100) reconstruction disappears with Sn exposures of 4.6 × 10¹⁴ atoms cm² (θ_Sn = 0.35 monolayer (ML)). Further Sn deposition results in a c(2 × 2) LEED pattern starting at a coverage of slightly above 0.5 ML. The c(2 × 2) LEED pattern becomes progressively more diffuse with increasing Sn exposure with eventual loss of all LEED features above 2.2 ML. Annealing experiments with various precoverages of Sn on Pt(100) are also described by AES, LEED, and LEISS results. For specific Sn precoverages and annealing conditions, c(2 × 2), p(3√2 × √2)R45°, and a combination of the two LEED patterns are observed. These ordered LEED patterns are suggested to arise from ordered PtSn surface alloys. In addition, the chemisorption of CO and O₂ at the ordered annealed Sn/Pt(100) surfaces was also examined using thermal desorption mass spectroscopy (TDMS), AES, and LEED.     

Chemisorption of Au on Si（001） surface

The chemisorption of one monolayer of Au atoms on an ideal Si(001) surface is studied by using the self-consistent tight binding linear muffin-tin orbital method. Energies of the adsorption system of a Au atom on different sites are calculated. It is found that the most stable position is A site (top site) for the adsorbed Au atoms above the Si(001) surface. It is possible for the adsorbed Au atoms to sit below the Si(001) surface at the B_1 site(bridge site), resulting in a Au－Si mixed layer. This is in agreement with the experiment results. The layer projected density of states is calculated and compared with that of the clean surface. The charge transfer is also investigated.