Atom probe studies of catalyst surfaces: structure and chemistry at atomic level

Abstract

The imaged (apex) region of a field ion microscope (FIM) specimen is sharply curved and has a radius of less than lOOnm. It is thus a reasonably good "model" for one half of a single particle of a metallic catalyst. FIM images show good detail of steps, ledges and kink site atoms (Fig. 1). When combined with a time-of-flight mass spectrometer to form an atom probe (AP), single atom chemical identification becomes possible. The FIM-AP combination has considerable value for the study of heterogeneous catalysts and catalytic reactions, but there are problems due to the high field acting on the specimens during observation, and the need to work in a vacuum environment. The most important applications to date have involved studies of the surface of the catalyst material, and of relatively non-labile adsorbates. However, new developments in AP instrumentation have opened the prospect of seeing catalytic reactions occurring on the atomic scale, and analysing the intermediate reaction products in both spatially- and time-resolved modes with high precision. Two main lines of development have contributed to this exciting prospect. Block and co-workers in Berlin produced the Pulsed Field Desorption Mass Spectrometer (PFDMS). In this instrument, a high electric field is initially applied to a FIM specimen in the presence of a reactive gas mixture. The specimen apex is cleaned by raising the field to a level sufficient to produce field evaporation, and then the field is dropped to zero to allow gas adsorption and reaction to occur.