## 3 Results and Discussion
### Effect of Catalyst Formulation on the Reduction Phase  
Lean/rich cycles at 150 \({}^{\circ}\)C and 300 \({}^{\circ}\)C are performed for the set of catalysts prepared (Figures 2a-d and 3a-e, respectively) to ascertain the influence of catalyst formulation. The mentioned Figures report H\({}_{2}\), H\({}_{2}\)O, NO, NO\({}_{2}\), NH\({}_{3}\), N\({}_{2}\)O, and N\({}_{2}\) outlet concentrations, along with the H\({}_{2}\) inlet profile (in dotted lines) of the rich phases corresponding to those cycles. The amounts of NO\({}_{\rm x}\) stored in the previous lean phase for every catalyst and adsorption temperature are compiled in Table S1 as Supplementary Data. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Cycle**} & **NO\({}_{\rm x}\) Adsorbed** & **Selectivity** & **Selectivity** & **Selectivity** & **Balance 1** \\  & **(\(\mu\)mol/g\({}_{\rm cat}\))** & **to N\({}_{\rm 2}\) (\%)** & **to NH\({}_{\rm 3}\) (\%)** & **to NO (\%)** & **N\({}_{\rm ads}\)/N\({}_{\rm pr}\)** \\ \hline Cycle 1 & 252 & 34 & 5 & 61 & 1.02 \\ Cycle 2 & 255 & 31 & 3 & 66 & 1.02 \\ Cycle 3 & 244 & 27 & 3 & 70 & 1.00 \\ Cycle 4 & 246 & 28 & 3 & 69 & 1.01 \\ Cycle 5 & 249 & 29 & 3 & 68 & 1.05 \\ Cycle 6 & 252 & 27 & 3 & 70 & 1.02 \\ Cycle 7 & 255 & 26 & 3 & 71 & 1.00 \\ \hline \end{tabular}  
* Relationship between the NO\({}_{\rm x}\) adsorbed (during the NO\({}_{\rm x}\) pulse) and the released N-species in the reduction pulse.  
\end{table} 
Table 1: Data from NO\({}_{\rm x}\) storage/H\({}_{2}\) reduction consecutive pulses (and final TPD), conducted at 300 \({}^{\circ}\)C for Ba6/Cu2 CZ. Figure 2: 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 150 \({}^{\circ}\)C, for the catalysts: (**a**) CZ; (**b**) Cu2/CZ; (**c**) Ba6/Cu2 CZ; and (**d**) Ba11/Cu2 CZ (Ba6/CZ not included due to very low catalytic response). At 150 \({}^{\circ}\)C, the Cu-free CZ catalyst does not exhibit activity (Figure 2a). The activity of the other catalytic systems is very poor, whatever the composition of the catalyst. However, a small H\({}_{2}\) consumption can be observed with simultaneous H\({}_{2}\)O production and NO generation (Figure 2b-d); indeed, very small amounts of NO\({}_{\rm x}\) were adsorbed onto the catalyst surface, thus being released in this rich step with the evolution of NO and water according to reaction (8), as already discussed. If Ba is not present in the catalyst formulation (Figure 2b), the cerium sites are the only sites responsible for NO\({}_{\rm x}\) adsorption (see Equations (6) and (7)); however, the evolution profile during the reduction step (NO and H\({}_{2}\)O emission) seems to be qualitatively similar. The activity of the Cu-free catalysts (i.e., CZ and Ba6/CZ) at 300 \({}^{\circ}\)C is also low (Figure 3a,c); indeed, small H\({}_{2}\) consumptions were registered in line with the low reducibility of these samples. As a matter of fact, dedicated H\({}_{2}\)-TPR experiments reported elsewhere [51] showed that the reduction did not take place in a reasonable extent until 400-500 \({}^{\circ}\)C. Moreover, a very small release of H\({}_{2}\)O is observed, which is mostly ascribed to a small partial reduction of these catalysts with a small generation of oxygen vacancies [51]. Only negligible amounts of NO and NO\({}_{2}\) were detected for CZ, and there is no evidence of other N-containing products.