# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Model Catalysts as Tips--Focus on the Surface
#### 3.1.1 Pt-Based Alloys  
Pt-based catalysts are used for various applications, among which the most important is the treatment of automobile exhaust gases, due to their low oxidation properties and stability at high temperatures [96]. A better understanding of the dynamic behaviors of alloys is required to fine-tune the surface composition of the catalyst, and improve its efficiency towards NOx reduction, CO oxidation and hydrocarbon oxidation. Different Pt-based alloys have been studied with a focus on the near surface, i.e., the very first atomic layers, with FIM methods allowing for a facet-specific analysis rather than a global analysis of the model nanoparticle. Pt-Rh samples, with a composition of Pt-17.4 at.% Rh, were studied after exposure to pure gases, mixtures of gases (either simultaneous or sequential exposure) and at different temperatures to understand the segregation behaviors. The specimens were produced by the electropolishing method from a Pt-17.4 at.% Rh wire in a molten salt mixture of 4:1 NaNO\({}_{3}\):NaCl (w:w) at \(\approx\) 520 \({}^{\circ}\)C. After pre-characterization, the specimens were exposed to different gases under field-free conditions to avoid any field-induced effects (field-induced morphological changes, field-induced dissociation... [97]). Segregation behaviors can be analyzed layer-by-layer and quantitatively using 'temperature-concentration plots'. Different segregation behaviors have been observed: for example, a Rh enrichment layer at the surface followed by a Rh depleted layer, Rh depletion at the surface, or even an enrichment layer turning into a depletion layer at a given temperature. The results [85; 88; 89] obtained on this Pt-Rh alloy can be summarized as follows:  
* the surface segregation behavior is facet-dependent due to different surface free energies and different diffusion processes;
* the surface segregation behavior is temperature-dependent, allowing observation of Rh surface segregation below a threshold temperature, and Rh surface depletion above this temperature;
* some oxygen-bearing molecules (NO, O\({}_{2}\) and N\({}_{2}\)O) cause surface segregation, whereas CO induces very little segregation, suggesting that the presence of atomic oxygen drives surface segregation in Pt-Rh;
* the competitive segregation effects due to different surface properties can be studied by exposing the specimen to different gases simultaneously or one after another, for example CO preventing the build-up of oxide layer and limiting the extent of segregation. A careful analysis shows that in some cases, the extent of the Rh enriched layer is not compensated by the Rh depleted region, signifying that multiple different diffusion processes are present. Vertical diffusion can occur, from the bulk to the surface, but so can lateral diffusion, from one facet to the other. In the case of Pt-Rh exposure to NO, lateral diffusion occurs from [001], [011], [012] towards [111] facets where the oxide-species are the most stable. This was later simulated by a model considering the transport processes normal and parallel to the surface [98]. The results obtained on Pt-17.4 at.% Rh were compared to experiments on Pt-23.9 at.% Rh-9.6 at.% Ru and Pt-17.3 at.% Rh-14.0 at.% Ir specimens [90], also produced with the same electropolishing methodology. Adding a third element to Pt-Rh catalysts improves the activity towards specific reactions, increases the resistance to poisoning or decreases the light-off temperature. On Pt-Rh-Ru alloys, the presence of Ru stabilizes the [111] surface since no segregation can be observed after exposure at increasing temperatures, even after longer exposures (1 h). On the {001} facet, Rh behavior is similar to the Pt-Rh case, i.e., surface segregation and surface depletion above a threshold temperature, but to a lower extent.