## 3 Results
Concerning the SEM-EDS analyses, only the sample coming from the thermic treatment under a humid oxygen atmosphere (sample **NSC_24_O2**) somehow displayed different results with respect to the pristine material. Indeed, from the EDS analyses of both samples, reported in Figure 5B, the presence of the expected elemental components of an NSC, namely Al, Ba, Ce, and Pt, was found, but the other precious metal Pd was not present in relevant amounts. As expected, after the thermal aging process was carried out for 24 h at 800 \({}^{\circ}\)C under O2 exposure, the oxygen peak remarkably increased the intensity, a predictable phenomenon that can be explained both by the formation of several oxides and by the adsorption of molecular oxygen on the active sites (Figure 5D). \begin{table}
\begin{tabular}{c c c c} \hline
**SAMPLE** & **T (800 \({}^{\circ}\)C)** & **Wet O2 Pretreatment** & **Time (h)** \\ \hline NSC\_8\_Ar & DONE & NONE & 8 \\ NSC\_24\_Ar & DONE & NONE & 24 \\ NSC\_8\_O2 & DONE & DONE & 8 \\ NSC\_24\_O2 & DONE & DONE & 24 \\ \hline \end{tabular} \({}^{1}\) Operating conditions as reported into the experimental section.  
\end{table} 
Table 1: Thermal aging tests of NSC slices \({}^{1}\). Figure 4: EDS analysis of the (**A**) upstream and (**B**) downstream samples of the NSC metallic catalysts. Similar to the case of ceramic catalysts, a comparison of the SEM images of pristine and thermally aged catalysts was not sufficient in terms of evaluating the entity of the aging process (Figure 5A,C). In contrast, the TEM technique proved to be much more effective in investigating the catalyst evolution, enabling the determination of the nanostructured Pt particle size distribution, a crucial parameter for establishing the NSC catalyst functioning. In fact, it is well known that a primary role of Pt in the NOx abatement process is the oxidization NO \(\rightarrow\) NO\({}_{2}\), which is also the first step of the NOx storage process. In addition, the proposed mechanism for storage invokes a reversible surface spillover step of NO\({}_{2}\) occurring between Pt and BaO sites. Therefore, to maintain a uniform distribution of Pt nanoparticles into the BaO matrix avoiding the sintering phenomenon is of crucial importance for achieving high rates of both NOx storage and its release [22]. A possible way to avoid Pt sintering is the control of the temperature, which should be maintained as low as possible (ideally lower than 600 \({}^{\circ}\)C), particularly when an oxidizing atmosphere is present [9]. To gain experimental evidence of these facts, TEM analyses were conducted on samples listed in Table 1, focusing attention on the Pt nanoparticle size evolution (Figure 6). TEM images, corroborated by statistical analyses (Table 2), pointed out remarkable modifications with respect to the pristine sample of Pt nanoparticle morphology and sizes after thermal treatment, particularly after prolonged exposure to oxygen. Indeed, while the Pt nanoparticles (dark spots in Figure 6) of samples **NSC_8_O2** and **NSC_24_Ar**, experienced only a slight increase in size, the sample heated for 24 h and exposed to a humid oxygen atmosphere (**NSC_24_O2**) underwent a dramatic growth of particles (Figure 6D). Figure 5: (**A**) SEM image and (**B**) EDS analysis of a pristine sample of NSC catalyst; (**C**) SEM image and (**D**) EDS analysis of **NSC_24h_O2**. Data reported in Table 2 highlight the growth of Pt nanoparticles from 8 to 43 nm as well as the broadening of their sizes distribution from 12 to 20%. These data clearly demonstrate that the sintering process is strongly accelerated by the oxidizing gaseous environments, with an increment of Pt particle sizes of more than 5 times in only 24 h of exposure. As catalytic performances of these NSC catalysts are also strongly dependent on structural factors of the washcoat, such as its crystal lattice and specific surface area, BET and XRD analyses were carried in order to highlight possible variations of this part of the catalyst induced by thermal treatment and oxygen exposure.