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 279

d n 9 r 4 n g | 8

Figure 10.16 Time-dependent behavior of the oxidation process in a Pt-23.9at%Rh-
9.7at%Ru alloy exposed to 1 bar of oxygen at 873 K showing preferen-
tial oxidation at a grain boundary (10 min); oxidation of the surface
and the bulk leading to an approximate MO 2 stoichiometry (30 min);
and separation of Rh-rich and Ru-rich oxides (90 and 300 min) which
remain separate phases after subsequent reduction.
(Reprinted with permission from ref. 106. Copyright (2012) American
Chemical Society).

.

To investigate the time dependent evolution of oxide formation of
Pt-23.9at%Rh-9.7at%Ru, the temperature of 873 K was selected 106 (an
intermediate value between Ru-rich and Pt-rich surface as highlighted in the
Pt-8.9at%Ru alloy). In this case, the whole sample was analyzed by LEAP
rather than examining individual crystal planes using the CAP. The treat-
ment consists of 1 bar of oxygen gas for either 10, 30, 90 or 300 min. Atom
maps resulting from these experiments are shown in Figure 10.16.
From the atom map after 10 min of treatment, the extent of oxidation is
more pronounced in the bulk rather than at the surface. The oxidation is
concentrated along a grain boundary (highlighted with the dashed line).
Such regions are known to be more prone to oxidation than grain interiors in
a wide range of materials. The grain boundary region appears enriched in
both Rh and Ru. In the regions immediately adjacent to the grain boundary,
Rh and Ru are correspondingly depleted. This is due to the high diffusion
coecient of species at grain boundaries, and explains why the initial oxide
formation appears at this particular feature. If no grain boundary is present,
the oxide occurs only at the surface as a thin layer after 10 min of treatment.
After 30 min, the apex of the sample presents a 10 nm thick oxide layer, and
the oxide continues to grow along the grain boundary. The analysis reveals
the presence of separate oxide regions rather than a uniform phase.