# Atom Probe Tomography for Catalysis Applications: A Review
## 4 Conclusions and Perspectives  
It is important for the interested researcher to note that the analysis of catalytic and electrocatalytic materials by APT is still an emerging field. The examples presented in this review highlight the current and future importance of atom probe tomography for catalysis applications. First, the analysis of model tip-samples allowed visualization of diffusion and segregation processes, as well as phase separations, giving a rational underpinning to the design of catalysts with specific core-shell structures, enrichments, and nanodomain formation. Moving to more realistic and more challenging catalytic materials, specific sample preparation procedures have been developed on a case to case basis: the electrophoresis method, the encapsulation method, powder solidification, electrodeposition, the use of fusible materials, and even the synthesis on an APT coupon followed by encapsulation, implemented to study oxide-support catalysts. The advances in FIB instrumentation, electron-beam-assisted-deposition methods and site-specific sample preparation techniques greatly assisted the development of such procedures. Of course, these methods have advantages and drawbacks, especially regarding the incorporation of contaminants, the time required, and the formation of regular/irregular surfaces, and there is considerable potential for improvements to increase the reproducibility and accuracy of the resulting data. Improved stability and decreased activity of iridium oxides towards the oxygen evolution reaction (OER) have been unveiled with a near-atomic-scale resolution by atom probe tomography, as well as the temporal evolution of species at the surface of iridium oxides, critical to better understand the composition/activity/stability relationships in electrocatalysis [203]. APT has also been used to better understand photocatalytic systems and the importance of the nanojunction and subsequent ion exchange between Cd chalcogenide and ZnO crystals [204]. Another striking demonstration is the use of isolated Ni atoms dispersed on graphene for the CO\({}_{2}\) reduction reaction (CO\({}_{2}\)RR). It is indeed known that for obtaining a high selectivity during CO\({}_{2}\)RR, catalysts with specific electronic structures are required [205]. Introducing transition metals into a known matrix could be used to tune the electronic properties of the metal [206], but also to pin the metal and avoid nucleation processes [207]. In this frame, Ni atoms embedded in graphene shells exhibit great catalytic CO\({}_{2}\)RR performance, and APT analysis provides a direct identification of the presence single Ni atoms [208], pushing the boundaries of resolution available by the APT technique. Analysis of unsupported nanoparticles has demonstrated the importance of the thickness of the shell in core-shell structures for the catalytic activity as well as the effect that impurities can have on the final surface composition and morphology. Experiments on supported nanoparticles showed that APT can be used as a quality control tool to validate the purity, the size, the composition, the dispersion, and the expected structure of synthesized catalysts. These specimens also suffered from the issues of heterogeneous materials in APT, leading to trajectory overlaps and aberrations, and the problem of porous supports, since porosity is present on unsupported catalysts as powders or as nanoporous structures. For the latter, APT proved its relevance to study the synthesis of nanoporous materials with tuned ligament and pore sizes. Finally, coke formation leading to the deactivation of zeolites can be traced back to a non-uniform distribution of Al within the bulk, leading to different acidic properties, and the nanoscale changes in elemental distribution can be used to assess the activity and stability of zeolite catalysts.