# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Porous Materials--Nanoporous Au
The second set of experiments uses an alternative procedure to fill the pores by electrodeposition [175; 176]. Electrodeposition of metals on a porous structure can lead to incomplete filling, clustered deposition and inhomogeneous filling [177; 178; 179]. Au\({}_{23}\)-Ag\({}_{77}\) materials were electrochemically etched in HClO\({}_{4}\) aqueous solution, and electrodeposition of Cu was performed from a CuSO\({}_{4}\) + H\({}_{2}\)SO\({}_{4}\) solution. Some regions of the specimen were not Cu-filled, due to deficient Cu concentration in the solution, but these regions were removed during FIB-milling (Figure 14a). The partial dealloying leads to the formation of a porous structure with \(\sim\)10 nm large ligaments and pores. Due to the differences in evaporation fields between Au and Cu, the field evaporation behavior is complex and leads to trajectory aberrations, and subsequently, to an overlap of Cu and Au + Ag phases at the interfacial regions. Despite this, a qualitative analysis (Figure 14b,c) shows compositional variation within the structures of one single ligament (black arrow in Figure 14b): the outermost part of the ligament is Au-enriched, whereas the core is Ag-rich with a composition at the center part close to the initial alloy composition. At the surface of the ligaments, some Ag-rich areas are also present, signs of a heterogeneous surface composition, which is an important finding since the functionality of nanoporous materials is affected by the ligament size, the local and the global chemical composition. It can also be concluded that electrodeposition is an efficient procedure to fill pores with diameters of \(\sim\)10 nm. We also note that further work used Ni to successfully fill the pores in npSi and that the correlation between electron tomography and atom probe tomography experiments in these systems allows for more accurate 3D reconstructions [180]. Further experiments were performed on npAu with small amounts of Pt, which is known to stabilize the nanoporous structure, to avoid coarsening in oxidizing environments, and to enhance the (electro)catalytic activity [181, 182]. These effects are supposed to be due to Pt segregation at step and kink sites where they can inhibit the mobility of step edges. Two alloys were studied using the same electrochemical etching and Cu electrodeposition procedure: Ag\({}_{77}\)Au\({}_{22}\)Pt\({}_{1}\) and Ag\({}_{77}\)Au\({}_{20}\)Pt\({}_{3}\), resulting  
Figure 14: Exploring the effect of Pt on the structure of npAu. (**a**) SEM image after lift-out (and before FIB milling) of a partially filled npAu sample by Cu electrodeposition; (**b**) 3D atom map of a npAu specimen filled with Cu; and (**c**) 1D concentration profile along the black arrow in (b) highlighting the core-shell ligament structure with Au-rich shell and Ag-rich core. Trajectory overlaps between the Cu and the Au + Ag phases leads to a rather wide interface; 3D reconstruction of (**d**) dealloyed Ag\({}_{77}\)Au\({}_{22}\)Pt\({}_{1}\) and (**e**) dealloyed Ag\({}_{77}\)Au\({}_{20}\)Pt\({}_{3}\) specimen showing different ligament and pore size due to the different Pt content. Panel (**a**): reprinted from [175], Copyright (2017), with permission from Elsevier. Panels (**b**–**e**): reprinted from [176], Copyright (2018), with permission from Elsevier. Figure 13: Self-supported nanoporous Au. (**a**) SEM image of the npAu structure (inset) and of a tip-sample after filling the voids by e-beam assisted deposition; (**b**) mass spectra from Fe-filled npAu revealing the presence of traces of contaminations and the absence of Ag; and (**c**) 3D reconstruction of a partial ligament and its interface with Fe-filling materials. Atomic depth resolution is obtained on Au. Reprinted with permission from [172]. Copyright (2015) Cambridge University Press. in npAu(Pt1) and npAu(Pt3). APT was used to explore the effect of Pt on the dealloying behavior and how the Pt concentration affects the morphology of the final structure [176; 183].