## 3 Results and Discussion
### Effect of Catalyst Formulation on the Reduction Phase
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{3}{c}{**NO Reduction (50 or 250 \({}^{\circ}\)C)**} \\ \hline
**Catalyst 1** & **Temperature (\({}^{\circ}\)C)** & \begin{tabular}{c} **NO Consumed** \\ **(\(\mu\)mol/g\({}_{\text{cat}}\))** \\ \end{tabular} & \begin{tabular}{c} **N\({}_{2}\) Produced** \\ **(\(\mu\)mol/g\({}_{\text{cat}}\))** \\ \end{tabular} & \
\begin{tabular}{c} **N\({}_{2}\)** \\ **(\(\mu\)mol/g\({}_{\text{cat}}\))** \\ \end{tabular} &
\begin{tabular}{c} **N\({}_{2}\)O/N\({}_{2}\) Ratio** \\ **Produced (\(\mu\)mol/g\({}_{\text{cat}}\))** \\ \end{tabular} \\ \hline Ba6/Cu2 CZ & 50 & 511 & 186 & 34 & 5.5 \\ Ba6/Cu2 CZ & 250 & 439 & 61 & 144 & 0.4 \\ Cu2/CZ & 50 & 784 & 315 & 72 & 4.4 \\ Cu2/CZ & 250 & 852 & 86 & 339 & 0.3 \\ \hline \hline \end{tabular}  
* Pre-reduction step under H\({}_{2}\) at 350 \({}^{\circ}\)C.  
\end{table} 
Table 2: Reaction data obtained during NO reduction reaction for pre-reduced Ba6/Cu2 CZ and Cu2/CZ. Figure 4: NO reduction experiment at 50 \({}^{\circ}\)C (**a**) and 250 \({}^{\circ}\)C (**b**) for pre-reduced Ba6/Cu2 CZ. According to these observations, it is proposed that during the pre-reduction in H\({}_{2}\), a population of oxygen vacancies is created; NO molecules subsequently interact with them, forming N-reduction products (see Table 2). It is worth noting that active sites linked to copper/ceria-zirconia's features are involved in the generation of oxygen vacancies. These, in turn, are able to reduce NO to N\({}_{2}\)O and/or N\({}_{2}\); NH\({}_{3}\) is not observed under these experimental conditions. The order of product observation is in line with the proposed reaction scheme, i.e., N\({}_{2}\)O before (reaction (12)), and eventually, N\({}_{2}\) (reaction (13)). In summary, for the fully formulated catalysts where both Cu and Ba are present, a considerable amount of NO\({}_{\rm x}\) is stored onto the catalysts as nitrites/nitrates. Stored NO\({}_{\rm x}\) is then reduced to NO, which is in turn reduced to N\({}_{2}\)O/N\({}_{2}\) (the latter being predominant) by its interaction with the oxygen vacancies; the selectivity to N\({}_{2}\) strictly depends on the degree of the catalyst reduction (i.e., Cu site reduction and the surrounding cerium centers), and therefore, on the population of oxygen vacancies that are created during the reduction step. Very minor NH\({}_{3}\) emissions detected in some experiments, (Figure 3d,e) could be explained by the occurrence of a global reaction (8), as previously anticipated. In conclusion, it is clear that H\({}_{2}\) consumption during the rich phase (Figure 5) is related to both the reducibility of the catalyst and the reduction of the stored NO\({}_{\rm x}\) species. To estimate this consumption, the storage step is followed by a reduction of one hour under isothermal conditions. The quantitative analysis is reported in Figure 5 for the different catalysts and temperatures. A very low reducibility is observed for bare CZ below 500 \({}^{\circ}\)C, as expected from the H\({}_{2}\)-TPR results reported elsewhere [48]. On the contrary, the reducibility of Cu2/CZ is very high at low temperatures, starting from around 150 \({}^{\circ}\)C. This behavior agrees with the H\({}_{2}\) uptake estimated in the H\({}_{2}\) reduction phase (see Figure 5), where CZ consumes a very low amount of H\({}_{2}\), and Cu-containing catalysts present higher consumptions. In all cases, the evolution of water is observed along with H\({}_{2}\) consumption, thus indicating that some catalyst reduction is taking place. Most of H\({}_{2}\) consumption can be correlated to H\({}_{2}\)O generation, but an accurate quantification of water is difficult and problematic in this case (as previously mentioned). In the H\({}_{2}\)-TPR results presented elsewhere [48], additional evidence is provided of the catalyst reduction that occurs when the temperature of H\({}_{2}\) feeding is high enough.