## 3 Results and Discussion
### Effect of Catalyst Formulation on the Reduction Phase
In the meantime, the stored NO\({}_{\rm x}\), present on the catalyst surface as nitrites/nitrates [51], are partially reduced to NO that is observed in the gas phase together with H\({}_{2}\) admission; indeed, as already observed, the catalyst is not sufficiently reduced to transform nitrites/nitrates into N\({}_{2}\), therefore only NO is produced. After a certain induction time, the catalyst reaches a sufficient reduction state and the released NO interacts with the oxygen vacancies being reduced to N\({}_{2}\), which is observed with a delay from the H\({}_{2}\) admission. [PERSON] et al. [52] explained the N\({}_{2}\) formation by the adsorption of two molecules of NO onto oxygen vacancies in close vicinity, and that their interaction yielded gaseous N\({}_{2}\) and the oxygen vacancies were filled with oxygen. On the other hand, [PERSON] et al. [53] described this N\({}_{2}\) formation with the generation of the hyponitrite species on an oxygen vacancy, by the adsorption of two NO molecules (reactions (10) and (11)); after that, they may decompose to N\({}_{2}\)O (reaction (12)), which can react again with an oxygen vacancy and yield N\({}_{2}\) and re-fill the oxygen vacancy (reaction (13)). This last mechanism could explain the very small production of N\({}_{2}\)O detected in Figure 3d. A tentative sequence of steps are presented below [54]:  
\[-\mathrm{Ce^{3+}-\square-Ce^{3+}-+NO\rightarrow-Ce^{4+}-[NO]^{2}-Ce^{4+}-} \tag{10}\]\[-\text{Ce}^{4+}-[\text{NO}]^{2-}-\text{Ce}^{4+}-+\text{NO}\rightarrow-\text{Ce}^{4+} -[\text{N}_{2}\text{O}_{2}]^{2-}-\text{Ce}^{4+}- \tag{11}\] \[-\text{Ce}^{4+}-[\text{N}_{2}\text{O}_{2}]^{2-}-\text{Ce}^{4+}- \rightarrow-\text{Ce}^{4+}-\text{O}^{2-}-\text{Ce}^{4+}-+\text{N}_{2}\text{O}\] (12) \[-\text{Ce}^{3+}-\square-\text{Ce}^{3+}-+\text{N}_{2}\text{O} \rightarrow-\text{Ce}^{4+}-\text{O}^{2-}-\text{Ce}^{4+}-+\text{N}_{2} \tag{13}\]  
In order to provide consistent evidence that support the occurrence of such plausible mechanisms of NO\({}_{\text{x}}\) reduction and pathways of formation for the N-reduction products exposed above, complementary NO reduction experiments are conducted with selected catalysts under simplified and controlled reaction conditions. The purpose of this fundamental study is to analyze how the NO molecules interact with the oxygen vacancies freshly created on the pre-reduced catalysts under H\({}_{2}\). The Supplementary Information compiles the description of these catalytic tests. Briefly, Ba6/Cu2 CZ is pre-reduced in H\({}_{2}\) at 350 \({}^{\circ}\)C, after which NO is admitted to the reactor at 50 \({}^{\circ}\)C or 250 \({}^{\circ}\)C; the corresponding profiles are reported in Figure 4 (Figure 4a,b, respectively). As can be seen, for the Ba6/Cu2 CZ catalyst, after NO admission at 50 \({}^{\circ}\)C (Figure 4a), a huge production of N\({}_{2}\)O and a lower amount of N\({}_{2}\) are observed, while the NO concentration curve increases slowly up to a steady state. The same behavior is observed after the NO reaction at a higher temperature (250 \({}^{\circ}\)C, Figure 4b) with faster kinetics in the reduction product formation. Table 2 summarizes the quantitative analysis performed in the case of pre-reduced Ba6/Cu2 CZ and Cu2/CZ catalysts as well. It clearly appears that the Ba-free catalyst is more reducible and higher amounts of N-products could be obtained with regard to Ba6/Cu2 CZ. Likewise, it is noteworthy that selectivity towards N\({}_{2}\) depends on the NO reaction temperature, being higher at a higher temperature, according to the sequence of reactions (10)-(13).