# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Powder  
The preparation of samples as powders has attracted attention since the early years of atom probe, and one of the important points is to prepare APT specimens without altering the microstructure of the materials. If the sample is large enough, i.e., micrometer-sized powder particles, and if pores are absent from the powder, a conventional lift-out procedure can be used without the use of embedding matrix [154; 155; 156]. Alternatively, a single powder particle can be mounted on a micro-post and then FIB milled [157]. If the size and porosity of the sample does not allow for conventional sample preparation, encapsulation within a tube can be used. The first attempt at this technique consisted of compacting the powder of interest in a Cu tube, cold-swaged down to a diameter of 200-300 nm; electrochemical etching was then used to sharpen the sample and analyze Cu-Ag particles by APFIM [158; 159; 160]. More recently, a glass-fiber micro capillary was filled with powder of steel/Ni/Cr, heated and extended with constant force fiber extractor to produce APT specimen suitable for analysis [161].  
In addition to the lift-out and tube encapsulation methods, a powder solidification procedure has been developed for the analysis of CoCuMn powder catalysts [162; 123]. Co-based materials are used for the Fischer-Tropsch reaction, i.e., the CO hydrogenation to produce long-chain hydrocarbons or alcohols [163; 164]. CoCuMn catalysts, prepared by co-precipitation of metal salts into oxalate precursors, allow for the production of long-chain 1-alcohols with high selectivity (up to 95%) while maximizing the yield of C\({}_{8}\)-C\({}_{14}\) terminal alcohols. Such catalysts are produced in the form of powders (Figure 12a), without the presence of a classical support. However, the voids within the powder needs to be replaced by an APT-compatible material to maintain structural integrity and avoid trajectory aberrations [79]. After dispersion of the catalyst on a substrate, an in situ micro-gripping system is used to pick up and manipulate an agglomerate of appropriate size (\(\sim\)2-3 \(\upmu\)m) [123]. The specimen is then forced onto a sharp micro-post (Figure 12b) and fixed with a layer of e-beam assisted deposition of Pt (Figure 12c-left). FIB pyramid milling (Figure 12c) and annular milling are applied to obtain the APT tip sample.  
The originality of the procedure is that the annular milling is performed under a constant flow of gaseous precursor while imaging the tip with the electron beam at high current and energy, inducing deposition of Pt within the voids between the particles. This produces a nearly void-free solid matrix for the specimen of interest, as can be seen on the final tip by SEM and TEM (Figure 12d) where two large particles are highlighted. For the powder lift-out, or powder solidification, procedure to be successful, a good adherence between the matrix and the particles is required, and a high purity matrix allows for improved quality data with fewer decomposition products in the mass spectra. The analysis of the specimen (Figure 12e) reveals its atomic structure with a Co-rich core and a Cu-dominated CoCuMn mixed shell (Figure 12f) responsible for the catalytic activity. This shell is \(\sim\)2 nm thick, and oxygen atoms are distributed at the core/shell interface.  
A similar catalyst, Co\({}_{4}\)Mn\({}_{1}\)K\({}_{0.1}\), also produced by co-precipitation, favors selectivity towards the formation of aldehydes [165]. The APT analysis of CoMnK post-reaction reveals the formation of a cobalt carbide phase (Co\({}_{2}\)C) and low concentration of Mn and K in the outer layers only [166]. These studies show that \"CoCuMn\" core-shell particles with tailored shell composition and \"CoMnK\" particles are highly efficient catalysts for the Fischer-Tropsch reaction, and only high-resolution techniques such as APT can accurately reveal the 3D structure and composition of the core-shell structures.