# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Unsupported Nanoparticles
#### 3.3.1 Imaging Core-Shell Ag@Pd Nanoparticles--Electrophoresis Method  
Hydrogen is an important energy vector in fuel cell technology, but the storage of pure hydrogen remains an issue [117]. To overcome this problem, one of the possibilities is to use stored hydrogen in the form of a liquid organic compound, which can then be released in situ, at low temperature and without the production of contaminants, such as CO, to avoid a loss of the fuel cell's efficiency. The decomposition of formic acid can lead to the release of hydrogen and has been used in liquid-phase transfer hydrogenation reactions [118]. However, the nature of the catalyst and the operating temperature are important parameters, and there is an absence of active and selective solid catalysts for formic acid decomposition at room temperature. The literature reports Pd as the most active metal for formic acid decomposition in aqueous phase [119], and the authors found that the activity is closely linked to the electronic properties of metallic nanoparticles, following the trend Pd \(>\) Rh \(>\) Pt \(\approx\) Ru \(>\) Au \(>\) Ag [120]. To further improve the catalytic process in terms of activity and cost, the synthesis of core-shell structures is a common strategy. Charge transfer between core and shell is known to modify the chemical properties of the surface. Pd-shell bimetallic nanoparticles with Ru, Rh, Pt, Ag or Au core have been synthesized, and it has been shown that Ag@Pd nanoparticles are the most active catalyst for formic acid decomposition at 20 \({}^{\circ}\)C. A more detailed analysis reveals that the thinnest Pd shell (with a 1:1 Ag:Pd ratio) is more active than thicker Pd shells (1:2 and 1:3 ratios). To confirm this, however, nanoscale characterization was necessary, so APT was used to characterize Ag@Pd nanoparticles with 1:1 and 1:3 ratios. The specimens were prepared using the electrophoresis method. The basic principle consists of depositing a droplet of the nanoparticles in solution on an electropolishing loop. Then, a voltage bias is applied between the loop and a Pt or Pt-Rh tip-sample, causing the nanoparticles to deposit on the tip-sample by electrophoresis (Figure 6a). The efficiency of the process depends on the nanoparticlessize, size distribution and polarity, on the voltage applied and its duration, and on the tip size. A typical value used in this work is 5-15 V\({}_{\text{DC}}\) for 10 s. Longer exposure leads to agglomeration of nanoparticles. The deposition is checked by electron microscopy (Figure 6b,c) prior to introduction in the APT instrument. APT measurement of 1:1 Ag@Pd nanoparticles is presented Figure 7. The atom map Figure 7a depicts Ag atoms as grey balls and Pd atoms as yellow balls: two distinct nanoparticles can be clearly distinguished using the isosurface method. A localized reconstruction of one of the particles is represented Figure 7b,c and shows a core-shell structure with a core almost entirely made of Ag, a shell rich in Pd extending over one to two atomic layers, and a sharp interface of less than 0.5 nm in width. Similar data reconstruction on the 1:3 ratio indicates a Pd shell five to 10 atomic layers thick. Correlating the catalytic activity with the nanoscale characterization suggest that the electronic promotion of Pd by the core Ag has a short range of just a few atomic distances. This electronic promotion leads to enhanced catalytic properties, proving the importance of nanoparticle engineering to obtain the optimum catalysts for the decomposition of formic acid. A similar electrophoresis method was used to study the structure and composition of CuZnGaO\({}_{\text{x}}\) catalysts for the in situ production of hydrogen from methanol [122]. Correlative in situ X-Ray diffraction (XRD), High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM)  
Figure 6: Sample preparation by electrophoresis.