Field ion microscopic studies of the CO oxidation on platinum: Field ion imaging and titration reactions

Elementary steps of the CO oxidation—which are important for understanding the oscillatory behavior of this catalytic reaction—are investigated simultaneously on different Pt‐single crystal surfaces by field ion microscopy. Due to preferential ionization probabilities of oxygen as imaging gas on those surface sites, which are adsorbed with oxygen, these sites can be imaged in a lateral resolution on the atomic scale. In the titration reaction a COad‐precovered field emitter surface reacts with gaseous oxygen adsorbed from the gas phase or, vice versa, the Oad‐precovered surface with carbon monoxide adsorbed from the gas phase. The competition of the manifold of single crystal planes exposed to the titration reaction at the field emitter tip is studied. The surface specificity can be documented in the specific reaction delay times of the different planes and in the propagation rates of the reaction‐diffusion wave fronts measured on these individual planes during the titration reaction with a time resolution of 40 ms. At 300 K the COad‐precovered surfaces display the {011} regions, precisely the {331} planes as the most active, followed by {012}, {122}, {001}, and finally by {111}. Reaction wave fronts move with a velocity of 8 Å/s at {012}, with ≊0.8 Å/s at {111}, and have a very fast ‘‘switch‐on’’ reaction at the (001) plane with 500 Å/s. At higher temperature, T=350 K, an acceleration of reaction rates is combined with shorter delay times. The titration reaction of a precovered Oad surface with COgas at T=373 K shows the formation of CO islands starting in the {011} regions with a quickly moving reaction front into the other surface areas without showing particular delay times for different surface symmetries. The two reverse titration reactions have a largely different character. The titration of COad with oxygen adsorbed from the gas phase consists of three different steps, (i) the induction times, (ii) the highly surface specific reaction, and (iii) different rates of wave front propagation. The reaction of COgas with a precovered Oad layer on the other hand starts with nucleating islands around the {011} planes from where the whole emitter surface is populated with COad without pronounced surface specifity

Field ion microscopic studies of the CO oxidation on platinum: Bistability and oscillations

The oscillating CO oxidation is investigated on a Pt‐field emitter tip by using the field ion mode of surface imaging of Oad sites with O2 as imaging gas. Based on data of the titration reactions [V. Gorodetskii, W. Drachsel, and J. H. Block, J. Chem. Phys. 100, C. E. UPDATE (1994)], external control parameters for the regions of bistability and of self‐sustained isothermal oscillations could be found. On a field emitter tip, oscillations can be generated in a rather large parameter space. The anticlockwise hysteresis of O+2 ion currents in temperature cycles occurs in agreement with results on single crystal planes. Unexpected regular oscillation sequences could occasionally be obtained on the small surface areas of a field emitter tip and measured as function of the CO partial pressure and of the temperature. Different stages within oscillating cycles were documented by field ion images. Oscillations of total ion currents are correlated with variations in the spatial brightness of field ion images. In the manifold of single crystal planes of a field emitter {331} planes around the {011} regions are starting points for oscillations which mainly proceed along [100] vicinals. This excludes the {111} regions from autonomous oscillations. With slightly increased CO partial pressures fast local oscillations at a few hundred surface sites of the Pt(001) plane display short‐living CO islands of 40 to 50 Å diameter. Temporal oscillations of the total O+2 ion current are mainly caused by surface plane specific spatial oscillations. The synchronization is achieved by diffusion reaction fronts rather than by gas phase synchronization

A low stray light, high current, low energy electron source

A design of an electron gun system is presented whose stray light emission is reduced by about three orders of magnitude compared to a regular low-energy electron diffraction gun. This is achieved by a combination of a BaO cathode run at rather low temperature and a 30° tandem parallel-plate analyzer used as an optical baffle. The system provides a high beam current of several microampers at 50 eV beam energy. The system can be used down to ∼10 eV

Kinetic oscillations and surface waves in catalytic CO + O₂ reaction on Pt surface: Field electron microscope, field ion microscope and high resolution electron energy loss studies

Recent experimental work has shown that the field electron microscope (FEM) and the field ion microscope (FIM) can also serve as an in situ catalytic flow reactor. Platinum field emitter (model catalyst, grain diameter ∼700 Å) expose different nano single-crystal planes with well-defined crystallographic orientations. Isothermal, non-linear dynamic processes of the CO + O2 reaction on Pt have been studied as well as the formation of face-specific adsorption islands, mobility of reaction/diffusion fronts and creation of chemical waves. Analysis of Pt surface with a local atom resolution of 4–6 Å shows the availability of a sharp boundary between the mobile COads and Oads fronts. It has been found that the back phase transition of Pt surface (100) hex ↔ 1 × 1 plays an initiating function in the generation of chemical waves along certain crystallographic orientations of Pt surface. The reaction zone maximum active in CO2 formation, with a width of up to 40 Å, has been discovered between adsorbed COads and Oads beds. The adsorption of O2 and the reaction of molecular O2ads2− state as well as an atomic Oads state with CO on the Pt(100)-(hex) and the Pt(100)-(1 × 1) single crystal surface have been studied with HREELS and thermal desorption spectroscopy (TDS) techniques

Second-harmonic generation and surface reconstruction: Cs on Ag(110)

The Cs induced reconstruction of the Ag(110) surface has been investigated by second-harmonic generation (SHG), combined with measurements of the work function change (Δφ) and thermal desorption spectroscopy (TDS). Adsorption of one monolayer of Cs at 100 K leads to a SHG amplification of 1100 compared to the signal level of the clean surface. Through annealing the sample, a local reconstruction can be obtained at temperatures between 165 and 220 K, as a precursor to the long range ordered (1 × 3) and (1 × 2) missing row reconstruction. The onset of the local reconstruction is identified by a sharp decline of the SH intensity. For θCs < 0.24 this can be understood as a variation in the conditions for the resonance of the radiation field with optical transitions between Cs derived resonant levels, caused by a modified adsorption geometry

Isotopic effects in field desorption of hydrogen from tungsten

The observation of a higher brightness of hydrogen FIM images compared to that of deuterium can be interpreted as being due to the different ion rates. By using a pulsed laser atom probe with an additional E × B mass separator and a dual ion detector the field ion rates of the different species of hydrogen as well as the yield by laser stimulated field desorption of surface species were simultaneously recorded. Under the same conditions (temperature, pressure, field strength, monitored area) the rates of hydrogen were a factor of √2 higher, the same ratio was observed for the field dissociation of molecular hydrogen. The binding energy in the field (~ 100 meV) was only slightly higher for hydrogen. In coadsorption experiments the field assisted reaction to H3 was much more pronounced than that of D3. Also, a strong isotopic effect was observed in the temperature dependence of this reaction