# Atom Probe Tomography for Catalysis Applications: A Review
## 1 Introduction  
Current trends in designing the size, shape and composition of catalytic nanoparticles to improve their selectivity and efficiency require understanding catalytic processes at the nanoscale. For this, it is of utmost importance to understand how the morphology and composition of the catalysts evolve during the reaction, the equilibrium-shape of the catalyst under reaction conditions, and the factors controlling the reliability of the process. For molecular analysis of a catalyst, high-resolution imaging techniques are necessary. Tomography techniques are particularly valuable to the study of both the surface and the bulk structure and composition of catalytic materials. Neutron tomography [1] and X-Ray microtomography [2; 3; 4] have only been used to a small extent to study catalytic materials due to their lower resolution, but there is growing interests in the study of catalysts by electron tomography [5; 6; 7] and atom probe tomography [8]. Atom probe tomography is part of the field emission techniques, which comprise the Field Emission Microscopy (FEM), Field Ion Microscopy (FIM) and Atom Probe Tomography (APT). These techniques can characterize materials with atomic scale resolution; one of the required conditions is to have specimens prepared as sharp tips since a high electric field is required to induce the relevant emissive behavior (either field emission, field ionization or field evaporation [9; 10]). FEM and FIM have been used to study model catalysts prepared as APT tip specimens: the reactions are performed under well-controlled reactive conditions and on catalysts of well-defined size,shape and morphology. Due to the tip-shape of the specimen, several facets of different crystallographic orientation are exposed at the same time, and the apex can thus be seen as a model of a single nanoparticle. FEM and FIM are used to image the surface before and after reaction, but also under reactive conditions, which makes these techniques particularly suited for the study of catalytic systems. This allows the characterization of the morphological reconstructions of the surface, as well as the dynamical processes occurring during the ongoing catalytic reactions. The studied systems consist mainly in reactions of interest for automotive pollution control on platinum group metal catalysts: CO oxidation [11; 12; 13], O\({}_{2}\) hydrogenation [14; 15; 16], NO hydrogenation [17; 18; 19] and NO\({}_{2}\) hydrogenation [20; 21; 22; 23]. There is however no direct chemical information available with FEM and FIM microscopes, and atom probe techniques were developed to overcome this issue. Early atom probe designs consisted in the coupling of a FIM microscope with a mass spectrometer, bearing the name of atom probe field ion microscopy (APFIM), 1-dimensional atom probe (1 DAP) or pulsed field desorption mass spectrometry (PFDMS). These instruments allow simultaneous in situ imaging and analysis by mass spectrometry of the adsorbate layer or the near-surface layers over a small and well-defined region of the specimen (\(\sim\)10 nm\({}^{2}\)). This permits the study of surface segregation in different alloys: Pt-Ir, Ni-Cu, Pt-Au, Pt-Rh and Pt-Ru [24; 25; 26; 27; 28]; the oxidation of metallic and alloy samples [29; 30; 31]; the adsorption and dissociation behaviors of different systems allowing the determination of kinetic parameters and energies of adsorption: N\({}_{2}\) and O\({}_{2}\)/Ni-Cu [32], CO/Mo, [33], S/Ni [34], cyanogen/Pt [35], CO/Ni [36; 37], NO and CO + H\({}_{2}\)O/Au [38; 39], CO\({}_{2}\)/Rh [40], and methanol/Rh for applications in direct methanol fuel cells [41; 42; 43]. Reactive behaviors can be studied in detail, including CO oxidation on Rh [44; 45; 46; 47; 48]; methanation on Rh and Ir [49; 50]; and NO hydrogenation on Pd and Au-Pd [51; 52].