## 3 Results and Discussion
### Effect of Catalyst Formulation on the Reduction Phase
It is suggested that the stored species are mainly decomposed to NO and/or NO\({}_{2}\), while they remain indifferent to the presence of H\({}_{2}\); this means that in the absence of Cu, H\({}_{2}\) is not activated and the catalysts do not reach an effective degree  
Figure 3: H\({}_{2}\), N\({}_{2}\), NH\({}_{3}\), NO, NO\({}_{2}\), and H\({}_{2}\)O outlet concentrations, and H\({}_{2}\) inlet profile (in dotted lines) of the reduction step under 2000 ppm H\({}_{2}\), at 300 \({}^{\circ}\)C, for the catalysts: (**a**) CZ; (**b**) Cu2/CZ; (**c**) Ba6/CZ; (**d**) Ba6/Cu2 CZ; and (**e**) Ba11/Cu2 CZ. of reduction, which promotes the NO\({}_{\rm x}\) reduction process. On the other hand, in the case of Ba6/CZ (Figure 3c), no significant release of reduction or decomposition products is observed, indicating greater stability of ad-NO\({}_{\rm x}\) species with respect to bare CZ. Different features are observed when Cu is present in the catalyst formulation (i.e., in the case of the Cu2 CZ and BaX/Cu2 CZ catalysts, \(X=6\), 11 wt %). Indeed, in these cases (Figure 3b-e), a huge amount of water is produced upon H\({}_{2}\) admission, which generates a high population of oxygen vacancies, in line with the extension of the reduction of these catalysts in the range of 150-300 \({}^{\circ}\)C, as observed during the H\({}_{2}\)-TPR experiments [48]. Considering the Cu2/CZ catalyst (Figure 3b), as soon as H\({}_{2}\) is admitted to the reactor, a very small and fast NO release is observed; the N-balance reveals that the very low amount of adsorbed NO\({}_{\rm x}\) (8.9 umol/gcat) is completely removed from the surface (see Table S1). The NO\({}_{\rm x}\) reduction is not selective to nitrogen, and only NO is detected during the first minutes of the reduction phase. It is suggested that the NO produced during the reduction step is the result of the stored NO\({}_{\rm x}\) reduction and not of a decomposition, as observed in the case of Cu-free catalysts, since huge amounts of water are simultaneously produced that indicate the occurrence of such reduction. Finally, by considering the fully formulated catalysts, i.e., Ba6/Cu2 CZ and Ba11/Cu2 CZ (Figure 3d,e) upon H\({}_{2}\) admission, NO is immediately observed, as in the previous case; however, after a very short period of time and in correspondence with the drop in NO concentration, N\({}_{2}\) evolves as well (together with very minor amounts of N\({}_{2}\)O). NH\({}_{3}\) concentration deviates slightly from zero, although rigorous quantification is somewhat difficult. Huge amounts of H\({}_{2}\)O are observed in both cases with a complex profile. The N-balance calculated in these cases is close to 1. An analysis of the previous results suggests a close analogy with traditional NSR systems. Indeed, for BaX/Cu2 CZ catalysts, NO\({}_{\rm x}\) is stored during the lean phase, forming nitrite and nitrate species as with the traditional Pt-Ba/Al\({}_{2}\)O\({}_{3}\) NSR catalyst; the stored species are then reduced during the subsequent rich phase. However, the selectivity towards N\({}_{2}\) of the Ba-Cu catalyst is definitely lower, with NO being the main product; on the other hand, the decrease in the N\({}_{2}\) selectivity of Pt-based catalysts is related to the production of NH\({}_{3}\) and this drawback should be overcome by exploiting it in a SCR downstream catalytic bed. The complex H\({}_{2}\) concentration profile and the delay in the N\({}_{2}\) emission suggests that before the catalyst becomes active in the stored NO\({}_{\rm x}\) reduction (i.e., before the production of N\({}_{2}\)) some degree of reduction of the catalyst is required (i.e., a consumption of H\({}_{2}\) is observed at the beginning of the reduction phase without the creation of N-reduced products). Indeed, at the beginning of the reduction phase, its partial consumption after H\({}_{2}\) admission is observed together with H\({}_{2}\)O production (Figure 3d,e); this could be explained by a catalyst reduction, which generates oxygen vacancies.