## 1 Introduction  
In recent decades, research has focused on the development of catalytic and photocatalytic methods based on the use of nanoparticles (NPs), which can be used in various applications related to chemical production as well as environmental technology. In the latter context, of wide interest is the reduction of greenhouse gases with special focus on the abatement of NOx using nanocatalysis [1; 2; 3]. The NOxs have manifold devastating effects on the atmospheric ecosystem (i.e., ozone layer depletion, acid rain etc.) and on human health. As emerges from several studies, the main contribution to NOx emissions comes from automobile sources, with almost half of all NOx produced [4]. The NOx storage catalyst (NSC) is one of the most promising concepts for the reduction of NOx from exhaust gas of lean burn combustion engines such as Diesel engines [5; 6]. In the NSC, there are precious metals (platinum, rhodium, palladium), various oxides, and other substances useful for NOx storage. The catalytic NSC converter is made of a ceramic (synthetic cordierite 2 MgO\(\cdot\)2 Al\({}_{2}\)O\({}_{3}\cdot\)5 SiO\({}_{2}\)) or metallic (chromium-iron-aluminum alloys) monolith, and its internal walls are covered with the washcoat, a mixture of several oxides such as BaO, ceria, zirconia, and \(\gamma\)-alumina, possessing a high specific area, thus improving the catalytic performances. The catalytic active phase, composed by one or more noble nanostructured metals, is uniformly dispersed on the washcoat surface and all these components exert a proper role that is based on a complex interaction among themselves [7]. Recent studies have demonstrated that the presence of barium in the washcoat is crucial for the NOx storage process, due to the reversible transformation of BaO (or carbonate) into BaNO\({}_{3}\), and that a great influence is also exerted by cerium [8]. For a Pt-BaO/Al\({}_{2}\)O\({}_{3}\) catalyst, [PERSON] et al. observed that the sintering of platinum particles increases exponentially with both the aging time and the temperature [9], and this not only decreases the conversion of the reactions but also affects the NOx storage process [10]. The sintering of both Pt particles and the washcoat phase leads to an inhomogeneous dispersion of the catalytic phase and lowers the number of the active sites for the NOx storage [11]. A further drawback of Pt sintering is the decrease in the contact surface between Pt and BaO, resulting not only in a lower activity for the NO oxidation but also in a reduced rate of NO\({}_{2}\) spillover from Pt to the neighboring barium adsorption sites, which in turn decrease the NOx storage activity [9; 10; 11]. Most of the previous works have shown that the sintering behavior of Pt is a function of time, temperature, and composition of gaseous atmosphere to which the catalyst is exposed [9; 10; 11; 12]. However, the sintering dependence on the gaseous environment has commonly been evaluated on model catalysts and has been based on NOx conversion efficiency, asserting that the oxidizing atmosphere tends to strongly increase the aging phenomenon [4; 12]. Similarly, thermal aging is a serious drawback of today's NSCs, imposing a high regeneration frequency of catalytic material to preserve the high levels of NOx conversion. To the best of our knowledge, few studies have been reported until now on real (commercial) catalysts for automotive applications, such as selective catalytic reduction (SCR) [13], three-way catalysts [14], and NSC catalysts [15; 16], aimed at establishing, with quantitative and morphological data, the separate contributions of thermal and chemical (oxidizing) effects to the aging process. In this context, based on our experience in organocatalysis and nanocatalysis [17; 18; 19; 20; 21], we envisaged that these contributions can be evaluated with the aid of electronic microscopy. Herein, we report studies on chemical and structural variations of the Pt particles and the washcoat of commercial NSCs ascertained via TEM and EDS analyses.