Nanosized metal particles representing the catalytically active phase of supported catalysts are hardly accessible for surface science methods. It is, therefore, beneficial to create well defined model systems which can be studied by surface sensitive techniques. The structurally heterogeneous surface of a field emitter tip can be considered as such a model system since it represents well the complex structure of a metal particle of supported catalyst. The tip surface, however, can be prepared and characterized with atomic resolution by Field Ion Microscopy (FIM). Using this and other field emission techniques (e.g. FEM, Li-FDM) one can observe in situ catalytic reactions on defined nanosized facets with a resolution close to 2 nm. Digitizing of the video-images allows processing of the reaction dynamics within the different virtual probe-holes arbitrarily located on the surface. The role of the atomic steps, size effects, diffusive coupling and microscopic fluctuations can be studied using O₂⁺ and Li⁺ ions as well as electrons as probing species. Monitoring the Li⁺ ions which are emitted from the probed surface area enables studying of the alkali-promoted reactions by utilising the ions of the promoter itself. In addition, the microscopic surface mobility of the promoter during the reaction can be examined in situ using the surface density fluctuation approach.
The probe-hole technique realised in the FIM mode allows also the mass-to-charge resolved potential analyses of the ions emitted from the few surface atomic sites. This renders possible the derivation of the binding energy of adsorbed molecules (e.g. CO and O₂ in CO oxidation reaction) in the particular surface atomic environment. The applied electrostatic fields which are much higher in the FIM (10 to 20 V/nm) than in the FEM or Li-FDM modes cause the local pressure enhancement near to the specimen surface permitting in this way the partial bridging of the pressure gap.

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