# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Model Catalysts as Tips
#### 3.2.1 Engineering the Surface Composition of Pt-Based Alloys  
The first example consists of Pt-22 at.% Rh samples which were prepared using the electropolishing method in a molten salt mixture. Those specimens were then oxidized under oxygen flow to study the  
Figure 4: Different segregation behaviors in Au-Ag tip-samples: (**a**) cumulative fraction diagram and (**b**) 3D reconstruction of O\({}_{2}\)-treated Au-Ag specimen (1 h at 373 K) showing Ag surface segregation (99.2 at.%) due to surface diffusion; (**c**) cumulative fraction diagram and (**d**) 3D reconstruction of N\({}_{2}\)O-treated Au-Ag specimen (0.5 h at 423 K) showing a Ag segregation region (41.8 at.%) tailed by a Ag depletion region (4.5 at.%) before recovering the initial bulk concentration due to bulk diffusion. Reprinted by permission from Springer: [92], Copyright (2018). formation of oxides [111]. The investigation shows that the oxide phase nucleates from the surface and then grows inwards. The concentration profiles not only reveal the stoichiometry of the oxide, M\({}_{2}\)O\({}_{3}\) (M = Rh + Pt), but can also distinguish different regions: a Rh-enriched oxide-phase (composition: with 35 to 40 at.% of Rh as compared to the initial 22 at.%), a Rh-depleted region; and the bulk with the nominal concentration. Using increasing oxidation temperatures (873, 973 and 1073 K) the kinetics of oxide growth can be studied, including the activation energy for Rh diffusion, and the diffusion coefficients at different temperatures can be measured. In this case, the diffusion of Rh from the bulk to the oxide/metal interface is the rate-limiting step in the oxide growth. Applying an oxidation/reduction cycle produces a thin layer of almost pure Rh at the surface (composition: 99.6 \(\pm\) 1.3 at.% Rh-0.4 \(\pm\) 0.1 at.% Pt), leading to a core-shell structure (Figure 5a) with a shell made of the most active and expensive metal. Similar oxidation and oxidation/reduction experiments were performed on Pd-6.4 at.% Rh samples [112]. The specimens were prepared by the electropolishing method in 10% perchloric acid and 90% acetic acid, followed by 2% perchloric acid and 98% butoxyethanol during a second electropolishing stage. Early stages of oxidation at 873 K lead to the formation of an oxide layer with PdO stoichiometry (composition: 46.6 \(\pm\) 0.2 at.% Pd-2.7 \(\pm\) 0.1 at.% Rh-50.7 \(\pm\) 0.2 at.% O). As oxidation time increases, the oxide layer evolves towards different isolated phases: Pd-oxide (PdO stoichiometry), Pd-Rh mixed oxide ((Rh\({}_{1}\)Pd\({}_{1}\))O\({}_{2}\) stoichiometry) and Pd-rich suboxide (Pd\({}_{2}\)O stoichiometry). Reduction in hydrogen efficiently removes the oxygen but does not significantly affect the distribution of metallic species. The isolated phases--or islands--are thus preserved leading to the formation of distinct Pd-rich and Rh-rich (composition: 54.1 \(\pm\) 2.0 at.% Rh-45.9 \(\pm\) 1.8 at.% Pd) clusters (Figure 5b). This provides a convenient way to synthesize multifunctional catalysts with nanoscale regions of differing composition in close proximity. Figure 5: Schematic representation of different behaviors observed upon oxidation and oxidation/reduction cycles allowing for a better engineering of nanoparticles. (**a**) Core-shell formation in the Pt-Rh system; (**b**) phase separation into nanoregions in the Pd-Rd system; (**c**) switch between Pt and Pd enrichment as a function of the temperature in Pt-Pd system; (**d**) switch between Pt and Ru enrichment as a function of the temperature in Pt-Ru system; and (**e**) phase separation in the Pt-Rh-Ru system. Adapted from [113], Copyright (2013), with permission from Elsevier and adapted with permission from [114]. Copyright (2012) American Chemical Society. A third set of experiments was performed on Pt-Pd alloy [113]. Pt-31 at.% Pd specimens were produced with the same electropolishing method as Pt-Rh specimens, and then oxidized for 5 h under 1 bar of oxygen flow at 673, 773, 873, 973 and 1073 K. Three different oxidation regimes were observed.