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 249

reconstructions can occur and modify the number of active sites, either prior
to the establishment of a steady state regime, or during the lifetime of the
catalyst as it ages. These reconstructions are driven by the minimization of
the total free energy of a system 2-4 and can be triggered by thermal and/or
chemical treatments. In addition to that complexity, alloys are very often
used in applied catalysis. At the microscopic scale, the surface composition
of the very first atomic layers often differs from that of the bulk leading in
some cases to extreme situations where a core-shell structure develops. This
surface enrichment is also affected by external thermal treatments and ex-
posures to reactive species, sometimes with opposing trends, making it pos-
sible to fine-tune the surface composition with a high degree of accuracy. In
systems where expensive noble metals are employed, surface segregation can
deliberately be engineered to reduce the total overall load of these metals. The
variations of the relative concentrations of metals usually affect mainly the
very first atomic layers of the surface. To control and to adjust these con-
centrations to desired values using atomically precise methods, experimental
techniques are necessary that are able to map the three-dimensional structure
of nanocrystals before and after physico-chemical treatments.
Field emission (FEM), field ion (FIM) and field desorption microscopies
(FDM) are analytical techniques based respectively on field electron, field
ion, and field desorption emission in a projection-type microscope. A sharp
tip sample points towards a screen, sometimes in the presence of a gas.
When the voltage on the tip is raised to adequate values, the electric field
allows for gas ionization or electron emission. 5 The charged particles are
accelerated towards the screen where their kinetic energy is converted into
light. These methods can thus be used to image the arrangement of surfaces
or adsorbed species at the surface of the extremity of a nanosized metal tip.
Well before scanning tunelling microscopy (STM), FIM was the first tech-
nique able to spatially resolve individual atoms in direct space by ionization
of gaseous species, 6,7 with a typical resolution of

d n 9 r 4 n g | 8

.

B

2 Å. It was developed
from FEM which is based on the emission of electrons in the presence of an
electric field, with a lower resolution of some 20 Å. Both FEM and FIM are
imaging techniques that—by contrast to STM—provide a complete picture
of the instant electron/ion emission distribution from the entire imaged
surface area. This allows the study of spatially correlated phenomena and
investigations of local surface reaction kinetics on an atomic-scale. 8-13 The
studied sample, the apex of a nanosized needle, exhibits a heterogeneous
surface formed by differently oriented nanofacets and can thus serve as a
suitable model for a catalytic particle of similar sizes. Figure 10.1 illustrates
this general idea. Figure 10.1a shows a transmission electron micrograph of
a supported catalyst. One single catalyst particle is modeled by the extremity
of a sharp tip (Figure 10.1b). A typical FIM picture of a metal tip (rhodium in
this case) is presented in Figure 10.1c, together with a comparison ball
model having the same crystal lattice (Figure 10.1d). Each ball of the model
represents one single atom. Specimens of the type shown in Figure 10.1c
usually serve as the starting point for experiments under conditions where