Ion Appearance Energy Spectroscopy Research Articles

Coadsorption of lithium and oxygen on W(112): Nanosized facets versus single crystals

Coadsorption of lithium and oxygen on a nanosized W-tip is studied using field ion appearance energy spectroscopy (FIAES). Binding energies of coadsorbed Li-adatoms are derived locally for chosen atomic sites on (1 1 2) facets for different oxygen and Li-coverages. Independently, the binding energies of Li-adatoms in coadsorbed Li/oxygen layers are determined for macroscopic W(1 1 2) single crystal samples from the adsorption isobars in adsorption-desorption equilibrium experiments and compared with the local nm-scale measurements. The comparison reveals a very good agreement of results obtained by two different methods on differing length scales.

High-field versus high-pressure: Weakly adsorbed CO species on Pt(1 1 1)

Adsorption of carbon monoxide on a nanosized apex facet of a [1 1 1]-oriented Pt field emitter tip was studied under high electrostatic field conditions (15–20 V/nm) by field ion appearance energy spectroscopy (FIAES). Weakly bound highly mobile CO species have been detected and their possible role in surface reactions such as CO oxidation on Pt was elucidated. The FIAES results are compared with those of vibrational sum frequency generation (SFG) spectroscopy of CO adsorption on a macroscopic Pt(1 1 1) single crystal, in a gas pressure range from UHV to atmospheric. SFG and FIAES are complementary in that SFG mainly provides information on strongly bound CO molecules, whereas FIAES nearly exclusively detects weakly bound species. Both high-field and high-pressure studies indicate the presence of a compressed CO adlayer including weakly bound species. The high-field mimics the high gas pressure, leading to similar high adsorbate coverage, which is required for the formation of weakly bound highly mobile CO species, which may be the active entities in CO oxidation.

Catalytic reactions on platinum nanofacets: bridging the size and complexity gap

In this short review we present few selected sections of our own work on the catalytic reactions as studied on a nanoscale using the field emission and field ion microscopies (FEM/FIM) and the corresponding probe-hole techniques: magnetic field ion mass separation combined with the field ion appearance energy spectroscopy (FIAES) and an atom probe with a Wien-filter ion mass selection combined with the time of flight (ToF) and coincidence analyses. We focus on the CO respectively hydrogen oxidation on Pt nanofacets present on an apex of a Pt tip that serves as a model of a single catalytic pellet of a supported catalyst. Exhibiting a heterogeneous surface formed by different orientations, like a catalyst pellet, a Pt tip can be prepared and characterized with atomic resolution. Surface reactions in such a well-defined system can be monitored with a resolution of ≤ 2 nm by FEM or FIM and the reacting species originating from few selected surface sites can be analyzed by FIAES or ToF in a probe-hole experiment. By using the single tip the usual averaging of data over at least few catalytic pellets of the common array-type model catalyst is avoided and thus a “smoothing out” of the characteristics of the single particles can be eliminated. As a result, the correlation of processes on individual nanofacets and fluctuation effects can be studied in situ. Until now, the FIM/FEM techniques are known to be quite successful in studying the oscillating surface reactions on Pt and Rh surfaces. In the present review we concentrate, in turn, on the local reaction kinetics in the nanosized reaction systems as mirrored by spectroscopic measurements and on the fluctuation-induced deviations from the behaviour predicted by macroscopic rate laws.

Field ionization of O 2 on oxygen-covered Pt(111) and W(111) as studied using field ion appearance energy spectroscopy

The local (probe hole) mass-to-charge-resolved retarding potential analysis of O 2 + ions originated from a few surface sites of the apex plane of a [1 1 1]-oriented Pt and W field emitter tips was performed. Applying a thermionic cycle, field-dependent values of the binding energies of O 2 molecularly adsorbed on Pt(1 1 1) and W(1 1 1) were derived from the field-ion appearance energies of O 2 + emitted at 79 K. Two different possible (noble-gas-like and field desorption) mechanisms of the field ionization of molecular oxygen supplied as imaging gas in the field-ion microscope (FIM) are considered on the basis of experimental data.

Compatibility of field emitter studies of oscillating surface reactions with single crystal measurements: catalytic CO oxidation on Pt

Kinetic oscillations in catalytic CO oxidation on Pt have been studied on large (millimeter size) single crystal planes of Pt as well as on a Pt field emitter tip that exposes different crystal facets of nanometer size. In order to examine the compatibility of results from the two types of experiments, the regions of different dynamical behavior (bifurcation diagram) have been mapped out in p CO, T-parameter space using a field electron microscope (FEM) and a field ion microscope (FIM). The comparison with the results of single crystal measurements shows that in the case of applied electrostatic fields less than 5 V nm −1 (FEM), the field-induced effects are negligible, but they are significant for fields exceeding 12 V nm −1 (FIM). The field-induced shift of the bifurcation diagram toward lower p CO values, observed with FIM, is explained in terms of a field-modified interaction of CO and O 2 with Pt studied here with field ion appearance energy spectroscopy. With coadsorbed lithium (submonolayer coverage), the existence range for rate oscillations is shifted toward higher p CO values. This shift is attributed to a redistribution of the electron density at the surface induced by alkali metal co-adsorption.

Molecular adsorption states of O 2 on W(111) at low temperatures (down to 5 K)

Adsorption of O 2 on W(111) at temperatures down to 5 K is studied using molecular beam (MB) and thermodesorption spectroscopy (TD) methods. The low-temperature TD spectra for the molecularly adsorbed O 2 on W(111) contain three peaks at 25, 27 and 60 K corresponding respectively to multilayer condensation, physisorption and weak molecular chemisorption. The role of the substrate structure for the O 2 chemisorption is discussed on the basis of present results for W(111) and earlier data for different metal surfaces. Field-effect on the O 2W(111) interaction is studied using field ion appearance energy spectroscopy (FIAES) of O + 2 ions emitted from the apex plane of a [111]-oriented W field emitter tip. Applying a thermionic cycle, field-modified binding energies for O 2 W(111) are derived from the O + 2 appearance energies measured at 79 K. FIAES data reveals a field-adsorbed O 2 layer as the origin of O + 2 field ions emitted from W(111) for field ion imaging.

Weakly chemisorbed CO layer on Rh(100) as detected by reverse flash and field ion appearance energy measurements

Using the reverse flash (RF) method, the sticking coefficient and binding energy of CO molecules on Rh(100) are obtained for CO surface densities up to 1.2 × 10 15 cm −2. The presence of weakly bound CO molecules with a binding energy of 0.34 eV is detected at n > 1.0 × 10 15 cm −2. Mass-to-charge-resolved retarding potential analysis of CO + emitted from Rh(100) is performed and absolute values of CO + field ion appearance energy obtained. By applying a thermionic cycle field-dependent values of the binding energy of CO Rh(100) at high coverages are obtained. A comparison of field-free (RF) results and field ion appearance energy spectroscopy (FIAES) data reveals the weakly bound (field-stabilized) CO layer as the origin of CO + field ions emitted from Rh(100) for field ion imaging.

Interaction of CO and O2 with Pt Studied by Field Ion Appearance Energy Spectroscopy

Interaction of CO and O₂ with Pt Studied by Field Ion Appearance Energy Spectroscopy

On the kinetics of oscillating reactions: [formula omitted] and [formula omitted] on platinum

Field ion microscopic and spectroscopic analyses have been carried out for individual, catalytically active sites of a Pt tip exposed to a continuous flow of H 2O 2 and H 2H 2O gas mixtures. Product ion yields (H 2O +, H 3O +) are measur function of H 2 pressure showing hysteresis and bistability; transitions between different reactive phases at distinct critical H 2 pressures are observed. For H 2/O 2/Pt, results agree qualitatively with previous (field free) experimental data and recent theoretical predictions thus indicating a minor influence of the applied electric field. For the oscillating H 2 H 2 O rea system the kinetics differs markedly. Local fluctuations and oscillating reaction diffusion fronts are imaged by probe hole FIM and are analyzed simultaneously using field ion appearance energy spectroscopy. Different surface-reaction pathways are found in different kinetic phases.

Noble-gas-like mechanism of localized field ionization of nitrogen as detected by field ion appearance energy spectroscopy

A local (probe-hole) mass-to-charge resolved retarding potential analysis of N 2 + emitted from a few surface sites of the apex plane of a [111]-oriented W tip is carried out and corresponding field ion appearance energies are obtained. A comparison of the results of N 2 + W(111) field ion appearance energy spectroscopy with independently obtained field-free nitrogen adsorption data reveals a noble-gas-like mechanism for the localized field ionization of N 2, supplied as imaging gas in the field ion microscope. The differences in the physical nature of the field-dependent terms of the appearance energy of N 2 + and CO + field ions are discussed in relation to the peculiarities of the high-field modified interaction of nitrogen and carbon monoxide with the tungsten surface.

Appearance energy spectroscopy of CO+ field ions emitted from W(111)

Field ion appearance energy spectroscopy of CO+ field ions emitted from a few surface sites of the W(111) plane was performed. Using a newly developed retarding potential technique, absolute values for the CO+ appearance energies were derived from mass-to-charge resolved retarding potential analyses of field ions of the carbon monoxide supplied as an imaging gas in the field ion microscope. From analysis of the CO+ field ion appearance energies, field ionization of CO on the W(111) surface was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on W(111) at high coverages were obtained. A comparison of the results for CO/W(111) with data for the field-free binding energy of CO on rough W planes, measured independently, is made. The mechanism of field ion imaging of metal surfaces with CO+ ions and the field dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.

Field ion appearance energy spectroscopy of CO + originated from Rh(111) and Au(111) surface step sites

Field ionization of CO at step sites of Rh(111) and Au(111) was studied with field ion microscopy and mass-to-charge resolved field ion energy spectroscopy. Using a newly developed retarding potential technique, absolute values for CO + field ion appearance energies were derived from measurements of the high energy onset of ion retardation curves and from in situ measurements of the work function of the retarding electrode. From analyses of the CO + field ion appearance energies, field ionization of CO on Rh and Au surfaces was found to proceed by field desorption. Applying a thermionic cycle, field-dependent values of the binding energy for CO on Rh(111) and Au(111) were derived. For comparison, CO field ionization was also studied at a passivated Rh surface. The mechanism of field ion imaging of metal surfaces with CO + ions and the field-dependent chemisorption of CO on different metal surfaces is discussed on the basis of experimental results and theoretical calculations.