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 273

Also, subsequent reduction of the oxide phase left a thin and almost pure
layer of Rh, suggesting a rather simple route for engineering the formation
of a core-shell structure. The addition of a third metal to the alloy will be
discussed in Section 10.3.3.2.

d n 9 r 4 n g | 8

10.3.2.1.3 Pd-6.4at%Rh: Formation of Nano-islands. Palladium-rhodium
alloys are used in catalytic converters for automotive pollution control.
The use of these alloys may present a good opportunity to generate two
separate active phases and to precisely adjust the load of PGM in the cata-
lytic converter. The time-dependent oxidation of Pd-6.4at%Rh was studied
at 873 K after exposure at 1 bar of oxygen for 10, 30 or 180 min. 90 After
10 min of treatment, two distinct regions of oxidation are observed: one at
the apex of the tip, and the other is composed of oxide islands at the side
of the specimen. Figure 10.10 shows the region of the sample analyzed by
the atom probe. The oxide present at the side of the tip results from the
migration of species from the specimen shank. The analysis of this oxide
has been performed with a ladder diagram (Figure 10.12a) and proximity
histogram (Figure 10.12b). With a bulk content of 6.4 at% Rh the surface
is slightly Rh-depleted (4.1 at%) and the underlying region is slightly Rh-
enriched (7.8 at%). The proximity histogram highlights the fact that the
oxide at the surface has a stoichiometry close to PdO, which is consistent
with previous studies. 91-93
The location of oxide on the shank of the sample is shown schematically
in Figure 10.13. Treatment for 30 min or longer leads to severe oxidation.
After 30 min, PdO remains the dominant oxide. However, the presence of
Rh-rich oxide is noticed close to the apex, with a stoichiometry for a mixed
oxide phase of (Rh 1 Pd 1 )O 2 which is quite unexpected at this temperature (as
compared to 1075-1125 K in other works 91-93 ). Such Rh-rich islands are
formed from within the PdO phase. Increasing the oxidation time to 180 min
leads to formation of the Pd-rich sub-oxide Pd 2 O. The analysis of a sample
oxidized at 873 K for 20 min and then subsequently reduced at 673 K for

.

Figure 10.12 (a) Ladder diagram of the Pd-6.4at%Rh alloy after 10 min of oxidation

at 873 K. (b) Proximity histogram at the oxide/metal interface.
(Reprinted with permission from ref. 90. Copyright (2012) American
Chemical Society).