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 280

The mixed oxide (enriched in both Rh and Ru) has a stoichiometry close to
MO 2 . After 90 min of oxidation, distinctly spatially separated Rh-rich oxide
and Ru-rich oxide regions become apparent. Treatment for 300 min in-
creases the level of oxidation with more extensive formation of separate
oxide phases with stoichiometries approximating to RuO 2 and Rh 2 O 3 . The
analysis of a sample oxidized at 873 K for 1 h and subsequently reduced at
673 K for 2 h in 1 bar of hydrogen highlights the presence of Rh-rich and Ru-
rich metallic phases, as also shown in Figure 10.16. The significant chemical
changes and phase separation during the oxidation process survive the re-
duction treatment. Nano-island structures with two separate active phases
may therefore be induced by an oxidation/reduction cycle, as was the case for
the Pd-6.4at%Rh alloy.
A comparison between Pt-22at%Rh, Pt-8.9at%Ru and Pt-23.9at%Rh-
9.7at%Ru alloys can be made. A cycle of oxidation (1073 K) þ reduction
(673 K) induces the formation of a Rh-shell in the Pt-22at%Rh alloy. In the
case of Pt-8.9at%Ru, oxidation treatments at temperatures below 873 K
generate the formation of very thin Ru-rich oxides, while above 873 K, only a
Pt-rich surface is present with little oxides apparent. The oxidation of Pt-
23.9at%Rh-9.7at%Ru at 873 K produces Rh-rich oxides and Ru-rich oxides as
distinct separate phases. Subsequent reduction at 673 K retains the separ-
ation as Rh-rich and Ru-rich metallic regions. Thus the ternary alloys exhibit
drastically different behavior, which cannot be predicted by respective
trends in the binary alloys. This opens up a wide range of new opportunities
for nanoscale engineering of catalyst surface compositions.

d n 9 r 4 n g | 8

10.3.3 APT Studies of Catalytic Nanoparticles
The previous section has demonstrated that APT is a powerful technique to
investigate the composition of alloy catalysts after exposure to reactive gases.
These types of studies are therefore providing a unique tool to study model
catalysts using the sharp sample tip as a particle approximation. The next
step towards a better understanding of catalytic nanoparticles lies in the
study of actual catalyst formulations used in practice. The size of catalytic
nanoparticles is usually less than 10 nm 112 and thus smaller than the apex of
field emitter tips (20 nm o R c o 100 nm). However, conventional tip samples
used in atom probe experiments can act as a support for these smaller
nanoparticles.

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10.3.3.1 Methods of Atom Probe Sample Preparation for
Nanoparticles
A detailed description of sample preparation for the APT analysis of nano-
particles is beyond the scope of this chapter, and thus only a brief explan-
ation is presented. Such developments are very recent and only two methods
have been reported: an electrophoretic method and a combination of
chemical vapor deposition (CVD) and focused ion beam (FIB).