# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Supported Nanoparticles
#### 3.4.2 Carbon-Supported Nanoparticles  
Another type of support commonly used for catalytic and electrocatalytic applications is based on high surface area porous carbon, and different C-supported commercial catalysts were analyzed by APT [121]: C-supported Pt, Pt/Co, and Ir@Pt nanoparticles. The samples were first dissolved in a methanol/water solution prior to sample preparation by electrophoresis. Several difficulties may arise in the analysis of such materials: the presence of a porous support; the number and location of particles influencing the evaporation rate across the specimen; and the evolution of the model geometrical shape during the experiment; three factors strongly influencing the accuracy of the 3D reconstructions. Optimized reconstructions were obtained by correlating the particles shape and size with TEM imaging. As a first set of experiments, Pt nanoparticles deposited on a porous C substrate were investigated and reveal a relatively complex morphology (Figure 11a) with Pt, C, C\({}_{x}\)H\({}_{y}\) and Na species detected (as well as traces of Cl). The nanoparticles appear to be enveloped in a layer of carbon-based materials, and thin slices of the 3D reconstruction not only show the position of Pt and C-based species in the  
Figure 10: Correlative APT-STEM study of Au/MgO system as supported catalyst. (**a**) HAADF-STEM image of the specimen before and after APT experiments to correlate the morphology of Au nanoparticles; (**b**) 3D atom map of the specimen analyzed by APT (O is represented as cyan, Mg as magenta and Au as yellow); (**c**) 3D maps representing only Au-rich regions (by applying the isosurface method); (**d**) individual cluster highlighting the presence of a diffuse Au/MgO interface and the presence of MgO within the Au nanoparticle; and (**e**) schematic representation of the Au protrusion leading to trajectory overlaps and local magnification. Panels (**a–d**): reprinted with permission from [150]. Copyright (2014) American Chemical Society. Panel (**e**): reprinted with permission from [149150]. Copyright (2012) Cambridge University Press. specimen (Figure 11b,c), but also reveal the presence of low-density regions, due to the porous nature of the support. A variation of the particle size as a function of the depth of analysis is observed and is in agreement with TEM experiments. An important observation is the presence of Na\({}^{+}\) ions in the 3D reconstruction, and control experiments were performed by probing (1) the Pt-Rh tip prior to any deposition (Figure 11d), (2) the tip after unsuccessful deposition of materials, in absence of Pt nanoparticles (Figure 11e); and (3) the tip after deposition of C-supported Pt (Figure 11f). In the first case, only Pt and Rh ionic species are detected, as expected. In the second case, water and C-based species are detected which originate from the carbon support, surfactant from the synthesis or fragment of methanol. In the last case, Na\({}^{+}\) ions are present in the mass spectra and the reconstruction reveals that they appear in association with C\({}_{x}\)H\({}_{y}\) species, not with Pt. The hypothesis is that these impurities were introduced during the original synthesis. Since such impurities may affect the catalytic activity and increase the nanoparticle's sintering [152], their presence should be avoided, and the washing steps should be improved. Pt/Co supported nanoparticles were analyzed with a similar methodology. With high loading of Pt/Co nanoparticles and possible agglomeration, the boundary between two distinct particles is not well-defined, which highlights the importance of good dispersion for such studies. For this analysis,  
Figure 11: Investigation of C-supported Pt nanoparticles.