# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Bulk Diffusion vs. Surface Diffusion on Au-Ag  
More recently, Au-Ag materials have been used as bifunctional catalysts: the minor Ag is active for oxygen dissociation and Au guides selectivity. Au-Ag materials are efficient catalysts for selective oxidation reactions [104; 105; 106; 107], but there is a lack of understanding of the activation processes leading to such efficiency. In this context, simple molecules are used to study the reaction dynamics [108] and the segregation processes [92] by field emission techniques.  
In particular, Au-Ag specimens [92], with a composition of Au-8.8 at.% Ag, were studied after O\({}_{2}\), N\({}_{2}\)O and H\({}_{2}\) gas exposure at different temperatures and 60 mbar pressure. The sample preparation was performed using the electropolishing method from a Au-8.8 at.% Ag wire, using an aqueous solution of 20% KCN with 2-4 V\({}_{\text{DC}}\). Similar to the previous Pd-Au case, thermal treatments for 1 h and at temperatures up to 573 K under vacuum conditions did not lead to any segregation behavior, in agreement with the estimation of Ag diffusion length. However, longer thermal treatments of 17 h at 373, 473 and 573 K lead to the observation of segregation extending over four to eight atomic layers. This suggests that, if given sufficient time for the diffusion to occur, Ag will preferentially segregate to the surface, which is explained by the lower free surface energy and lower molar heat of sublimation of Ag as compared to Au [109; 110]. When the specimen is exposed to oxidative gases, a different Ag surface segregation occurs (see Figure 4). In the first case, O\({}_{2}\) induces a substantial surface segregation in terms of Au to Ag ratio, with surface concentration up to 99.2 at.% Ag after 1 h of treatment at temperatures as low as 323 K. In the second case, N\({}_{2}\)O exposure also induces Ag segregation, but to a lower extent, with a maximum of 41.8 at.% Ag after 1 h of treatment at temperatures starting from 373 K. As expected, 17 h of treatments leads to greater segregation behavior. Figure 4 represents the cumulative fraction diagram and the 3D reconstruction of the O\({}_{2}\) and N\({}_{2}\)O experiments. The cumulative fraction diagram plots the total number of Au and Ag atoms detected as a function of the total number of Ag atoms detected. The slope of such plots corresponds to the composition of the analyzed specimen, and any changes indicate segregation or depletion behavior. In the O\({}_{2}\) case (Figure 4a,b), Ag segregation is observed at the surface, followed by the nominal composition of the bulk. However, in the N\({}_{2}\)O case (Figure 4c,d), the Ag segregation region (41.8 at.% Ag) is directly followed by a depletion region (4.5 at.% Ag), and then the nominal composition of the bulk is recovered. The segregation behaviors are due to surface diffusion after O\({}_{2}\) exposure, and to bulk diffusion after N\({}_{2}\)O exposure: it is suggested that the different dissociation properties of these gases explain this difference. Finally, early results on the possible Au surface segregation have beenreported, exposing the specimen to reducing atmosphere (H\({}_{2}\) exposure) at 323 and 373 K for 1 h, which is likely to be due to field-induced surface diffusion of gold during the analysis.  
Experiments on both Pd-Au and Au-Ag further show that the composition of the surface can be fine-tuned by applying different physico-chemical treatments and by controlling the gaseous environment, the temperature and the length of the treatment.