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 282

10.3.3.1.2 Preparing an APT Tip from a Catalyst Powder: CVD-FIB
Method. This method has been successfully applied to produce atom
probe specimens from powders, 115-117 and more recently, from CoCuMn
particles. 118 The alloy is synthesized in the absence of a support and is
produced in a porous form having a very high specific area of some
170 m 2 g 1 . The first step of specimen preparation require the use of elec-
tron beam assisted chemical vapor deposition (eBCVD) of methylcyclo-
pentadienyl (trimethyl) platinum( IV ) in order to fill the pores in the
catalyst structure. This fully dense sample is then shaped by focused ion
beam (FIB) milling with an accelerating voltage of 10 kV. A final clean-up
of the sample is made by FIB with a lower voltage of 5 kV.

d n 9 r 4 n g | 8

10.3.3.2 Analysis of Unsupported Nanoparticles
APT has been successfully used to image and characterize the core-shell
structure of nanoparticles in cases where diffraction techniques and TEM
were unsuccessful, either due to the ultra-thin shell broadening the X-ray or
energy-loss peaks, or due to the lack of atomic-species contrast. This section
reports on the study of two nanoparticle systems used for specific catalytic
reactions: Ag@Pd core-shell nanoparticles for hydrogen production from
formic acid, and CoCuMn nanoparticles for CO hydrogenation. APT results
provide a better understanding of the activity and the eciency of such
catalysts for these particular reactions.

10.3.3.2.1 Ag@Pd Used in Hydrogen Production. Ag-Pd core-shell nano-
catalysts have been used for production of hydrogen from formic acid at
ambient temperature. The formation of Ag-Pd core-shell structures is
achieved by wet chemical synthesis. Details of the synthesis can be found
elsewhere. 113 The aim is to enhance the catalytic activity towards formic
acid decomposition at room temperature, essential for small mobile fuel
cell devices, through the retention of Pd atoms at the surface. The activity of
these can be enhanced via electronic interactions with a suitable core in a
core-shell bimetallic particle. 119,120 APT has been used to confirm the core-
shell configuration and to study the thickness of the Pd-shell as a function
of the Ag : Pd ratio used for the wet-chemistry synthesis route.
The samples were produced by the electrophoresis method on Pt-22at%Rh
support substrates of 80-100 nm diameter (dipping conditions: 5-15 V for
10 s). The 3D reconstruction of each individual particle is sliced for atomic
depth profiling. Figure 10.18a presents an atom map of a single Ag@Pd
nanoparticle, where the core-shell structure can be clearly observed. The
inner core is composed of Ag (gray dots) and the shell is composed of Pd
(yellow spheres). The interface between the two phases is very sharp. The
thickness of the shell regions can be analyzed. In the case of a 1 : 1 Ag : Pd
ratio, Pd shells are restricted to thicknesses of 1 to 2 atomic layers, as seen in
Figure 10.18. In the case of a 1 : 3 Ag : Pd ratio a broader shell of 5-10 atomic
layers is measured (not shown). Higher catalytic activity of the 1 : 1 ratio

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