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 269

enrichment in Rh is expected in Pt-17.4at%Rh alloy after exposure to oxi-
dizing gases such as NO and O 2 , owing to the favorable Rh-O bond strength
over Pt-O. 75 The focus will be on {111}, {001}, {011} and {012} crystallo-
graphic orientations using the smaller field-of-view CAP. When Pt-
17.4at%Rh is exposed to NO gas during 15 min at a pressure of 10 mbar at
increasing temperatures, a general observation is that the number of NO
species detected by mass spectrometry decreases as the number of atomic
oxygen species increases. At 573 K, few oxygen species are left on the Pt-
17.4at%Rh surface except on the {111} regions which desorb significant
amounts of RhO and PtO species. On {111} regions, the exposure of NO at
increasing temperatures also leads to an increasing segregation of Rh in the
very first atomic layer (up to 42 at% at 573 K). The second atomic layer
suffers from depletion in Rh species, suggesting the occurrence of diffusion
from the bulk to the surface. However, the Rh enrichment exceeds the Rh
depletion of the underlying atomic layer. To understand this, an analysis on
a wider total area of the surface is necessary. The atom maps established on
the {001} regions show a more complex behavior. Rh enrichment is observed
after NO exposure below 523 K. This Rh segregation turns into Rh depletion
at 573 K and above, with less than 4 at% of Rh at the surface. A careful
analysis of the corresponding atom map shows that Rh is present only at the
edges of the facet. A plausible explanation is the surface diffusion of Rh away
from {001} facets, which explains the depletion of {001} regions as well as the
greater enrichment of the {111} regions. Accordingly, the reported surface
enrichments result from a complex combination of atom displacements
along directions parallel and perpendicular to the surface. The {011} facet is
equally complex: surface segregation is observed up to 423 K, beyond which
a gradual reduction in surface Rh content with further temperature rises is
noted, leading to severe Rh depletion (less than 3 at% at 673 K). Regarding
the {012} facet, the combined first and second layers show Rh enrichment up
to around 473 K - the trends are less clear here probably because of the
inherent roughness of the {012} facets. Increasing the temperature induces a
gradual loss of Rh from the first layer, down to a minimum value at 673 K.
These results are consistent with the FIM micrographs of the sample
showing flat {001} and {012} surfaces.
Similar trends are obtained in the case of O 2 and N 2 O exposure. Rh en-
richment increases on the {111} facets up to a maximum of 30-35 at%,
while Rh depletion is observed on the {001} surface after exposure at 573 K.
These experiments indicate that the presence of free oxygen atoms drives
surface segregation in Pt-17.4at%Rh. A greater apparent activity is observed
in the case of NO exposure as compared to O 2 on the same surface, which
may be due to a higher NO sticking coecient.
In blank experiments, heating the sample at 823 K under UHV conditions,
or exposing the sample to N 2 or C 2 H 2 (at respectively room temperature and
423 K) did not induce any segregation (similar results were also reported in
ref. 76 and 77). A model for such oxidation-driven surface segregation on Pt
alloys has also been proposed. 42

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