# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Porous Materials--Nanoporous Au  
As previously mentioned, voids or deprocessed materials needs to be filled with APT-compatible materials for a better accuracy of the APT data [164]. This becomes extremely important for self-supported nanoporous catalysts where the porosity can be as high as 70%. Catalysts in the form of nanoporous bulk materials have recently attracted much attention due to their high surface-to-volume  
Figure 12: Sample preparation and resulting APT experiment of CoCuMn catalyst as powder. (**a**) particle agglomerate; (**b**) in situ micro-gripper used to force the catalyst on a sharp micro-post; (**c**) e-beam assisted Pt deposition and pyramid milling; (**d**) final smooth and void-free tip morphology imaged by SEM and TEM (two particles of 20–30 nm are highlighted); (**e**) top view of the APT detector during APT experiment; and (**f**) 3D reconstruction of a particle with core-shell structure (Cu is depicted as orange, Co as blue, Mn as green and O as white). Panels (**a–d**): reprinted from [123], Copyright (2015), with permission from Elsevier. Panel (**e**): reprinted with permission from [79]. Copyright (2017) Cambridge University Press. Panel (**f**): reprinted with permission from [162]. Copyright (2013) American Chemical Society. ratio, their mechanical stability, their porosity allowing for mass transport of reactants and products, their absence of support, and most of all, due to their activity towards oxidation reactions [104; 105; 106; 107; 107; 168; 169]. These structures are produced by dealloying methods, i.e., the selective etching of the least noble metals from an initial alloy [170; 171], and the chemical nature, concentration and nanoscale spatial distribution of the residual elements greatly influences the dealloying kinetics, the morphology and the catalytic activity; nanoscale characterization is thus required. To investigate such open-cell structures and avoid overlapping ion trajectories and distorted reconstructions, specific sample preparation methods have been developed using electron-beam assisted deposition within the FIB system and using electrodeposition. The first example consists of the analysis of Au\({}_{30}\)-Ag\({}_{70}\) materials dealloyed in HNO\({}_{3}\) by free corrosion [172], generating a nanoporous gold sample--npAu--with pores of \(\sim\)50 nm in diameter (Figure 13a--inset). Two precursors were used to fill the pores and get a compact material: Co\({}_{2}\)(CO)\({}_{8}\) and Fe\({}_{2}\)(CO)\({}_{9}\), both precursors yielding relatively high purity metals as compared to more classical precursors (Pt- and W-based). Several parameters are used to optimize the penetration and filling depth of the metals into the high-aspect ratio npAu: beam acceleration voltage, beam current, dwell time, pattern type and deposition time. Further FIB-milling produces suitable specimens for APT analysis (Figure 13a). The analysis shows that both Co- and Fe-based fill yields reliable APT data when acquired using voltage-pulse mode, with Fe-based precursor yielding more stable specimens. In Figure 13b, the mass spectra resulting from Fe-filled npAu shows the presence of the expected Au, Fe-based species, but also the incorporation of H\({}_{2}\)O into the deposit, probably due to contamination during transfer. Interestingly, Ag is absent from the mass spectra. A 3D reconstruction (Figure 13c) reveals that atomic planes, separated by a distance of 0.23 nm corresponding to Au[111] planes, can be observed, highlighting the high resolution of the technique. The analysis of the interface proves that there is no significant local variation in composition. The authors also mentioned that Pt deposition precursors do not allow for stable APT experiments under voltage-pulse mode. Further work proved that Pt-based precursor as a filling material allows for npAu analysis by APT in laser pulse mode [173; 174], emphasizing the importance of acquisition parameters.