Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 252

resolution. The three-dimensional atomic-scale reconstruction technique
has become known as atom probe tomography (APT). This makes possible
the establishment of the relationships between the structure of a material,
down to the atomic level, with its macroscopic features (ductility, hard-
ness, 25 magnetism, 26 radiation damage, 27 etc.). Today, these atom probes are
recognized as mainstream microscopy tools and are the subject of a dra-
matic increase of interest, evidenced by a recent increase in the number of
publications from academic and industrial laboratories that now have such
facilities. For catalysis studies, these atom probes can unravel subtle
variations of the surface and bulk composition of catalyst particles after
exposure to reactive environments.
This chapter will first outline the basic principles of APM. Subsequent
sections will address selected examples of its application in catalysis studies
with an increasing level of complexity, as follows: (i) adsorption studies of
single gases on single metal tip samples, (ii) the imaging and the chemical
probing of nanosized tips during catalytic reactions; (iii) the study of seg-
regation effects of alloy tips samples after physico-chemical treatments, and
(iv) APT studies on single catalyst particles extracted from supported cata-
lysts used, amongst others, for the Fischer-Tropsch reaction. This bottom-
up approach will aim to show the value of APM for the development of solid
catalysts with an atomic level of precision.

d n 9 r 4 n g | 8

10.2 Imaging and Local Chemical Analysis of

Nanosized Crystals Before, During and After
Catalysis

.

10.2.1 Sample Preparation
All APMs require the presence of an intense electric field of the order of
2-50 V nm 1 . When the voltage on the tip is raised typically to several
kilovolts, these values are achieved by field enhancement at the highly
curved surfaces. 5 For this reason, specimens are shaped as a sharp needle
tip, the extremity of which is the feature of interest with typical radii of
curvature of 5-100 nm. The production of such samples is thus an important
aspect of the experimental procedure. Extended and recent reviews on
sample preparation are available for a wide variety of materials. 18,28 We will
address here those relevant for catalysis studies.

10.2.1.1 Electropolishing Methods
Electrochemical polishing was historically the very first method to produce
tip samples from wires or 'matchsticks' of bulk materials and is still very
widely used. An electrochemical cell is built so as to progressively dissolve
the outermost material of the wire in a controlled manner when appropriate
conditions are used. The identification of optimum conditions relies on