# Atom Probe Tomography for Catalysis Applications: A Review
## 3 Case Studies
### Catalysts as Porous Materials--Zeolites  
Zeolites are a class of microporous crystalline materials made from aluminosilicates. With their high surface area, tunable pore and channel sizes, adjustable acidity/basicity and ion exchange properties, zeolites are widely used as catalysts [184; 185; 186; 187]. Due to the different charge of Si and Al units, the Si/Al ratio within the zeolite guides the acidity of the zeolite, and subsequently, its activity and selectivity. Determining the quantitative spatial distribution of active sites is therefore of utmost importance to better understand the reactivity of this class of catalysts, and APT is able to provide such information. As described in [188], the critical aspects of zeolite material analysis by atom probe are, among others:  
* the sample preparation, due to heterogeneous distribution of elements at the microscale;
* the proper assignment of mass spectra peaks, due to the complex fragmentation patterns of carbon-based materials and the lack of fingerprint fragmentation patterns in APT;
* the limitation of spatial resolution, blurring the atomic positions;
* the proper statistical data analysis. In searching for chemical heterogeneities to assess the structure-composition-reactivity relationships, different data treatments can be used: iso-concentration surfaces and 1D-concentration profiles to visualize phase separation; nearest neighbor distributions to discover the presence of precipitates and their short-range distributions; radial distribution functions to examine affinity between species, especially at short distances; and cluster analysis to isolate clusters (down to 10 atoms) within a matrix [188]. The first APT analysis of zeolites consisted of the ZSM-5 crystal (Figure 15a) before and after steam treatment [189], which is a common post-synthesis process to either stabilize or deactivate the ZSM-5 zeolite [190]. The specimen was prepared by conventional lift-out method (Figure 15b). However, Ga-ion beam damage leads to the formation of an amorphous phase, and thus low ion beam current is necessary, even though the outermost part of the tip sample remains amorphous (Figure 15c,d). Before any treatment, the distributions of Si, O and Al within the zeolite are rather homogeneous (Figure 15e). After steam-treatment of the specimen, the Al distribution becomes heterogeneous, with formation of Al-rich clusters within the bulk, and segregation at a grain boundary (Figure 15f--the blue and yellow dots on this figure represent the protective layer deposited for FIB sample preparation). Further distribution analysis reveals a shorter most probable distance and higher Al density in the steam treated sample (18 \(\pm\) 6 A and 0.025 atoms.nm\({}^{-3}\) in the untreated sample vs. 0.5 atoms.nm\({}^{-3}\) and 9 \(\pm\) 3 A in the treated sample). ZSM-5 crystals were also studied after partial deactivation due to methanol-to-hydrocarbons reaction (after 90 min of reaction at 623 K) using \({}^{13}\)C-labeled methanol to assign the presence of \({}^{13}\)C to the reaction rather than any other sources [191]. The coke formation resulting from this reaction is known to emerge from the formation of polycyclic aromatics near channel intersections and pore openings, leading to a blockage of the pores and deactivation of the zeolite [192; 193]. Using cluster analysis and iso-concentration surfaces, the presence of \({}^{13}\)C agglomerates, with a median size of 30-60 \({}^{13}\)C atoms, was revealed (Figure 15g), indicating the presence of several aromatic cycles, even though the APT technique is unable to provide molecular fingerprinting information of the \({}^{13}\)C clusters detected. It was also discovered that, within the \({}^{13}\)C clusters, the Al concentration was significantly higher than in the bulk, leading to an increased Bransted acid site density. The coke formation can therefore be linked to the Al concentration, which reflects the acidity, down to the nanoscale. Such a study provides important insights into the coke formation mechanism.