## 3 Results and Discussion
### Thermal Desorption of Stored NO\({}_{x}\)  
In order to check the complete removal of stored NO\({}_{x}\) from the surface after the reduction step, i.e., to verify the effectiveness of the H\({}_{2}\) reduction under these experimental conditions, a thermal decomposition was performed, heating every catalyst up to 500 \({}^{\circ}\)C in inert atmosphere. The corresponding concentration profiles are reported in Figures 7a-d and 8a-e, referring to results after adsorption-reduction cycles at 150 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, respectively. Figure 6: In situ DRIFTS spectra of CZ (**a**) and B46/CZ (**b**) during the reduction step in H\({}_{2}\), at 300 \({}^{\circ}\)C.  
In the case of the Cu-free sample (CZ) (Figure 7a), the decomposition of NO\({}_{\text{x}}\) stored at 150 \({}^{\circ}\)C mainly yields the evolution of NO and O\({}_{2}\), and minor amounts of NO\({}_{2}\), in line with results reported in a previous work [54]. For this catalyst, NO is seen desorbed in two peaks, a small one at a low temperature (max near 230 \({}^{\circ}\)C) and a larger one at a high temperature (close to 415 \({}^{\circ}\)C, see Table 3). \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Cycling Temperature**} & \multicolumn{2}{c}{**NO\({}_{\text{x}}\)/O\({}_{\text{2}}\) Ratio and T\({}_{\text{max}}\) (\({}^{\circ}\)C)**} \\ \cline{2-3}  & **CZ** & **Ba6/CZ** \\ \hline
150 \({}^{\circ}\)C & 2.8 (415) & - \\
250 \({}^{\circ}\)C & 2.8 (427) & 1.4 (444) \\
300 \({}^{\circ}\)C & 1.6 (426) & 1.4 (445) \\
350 \({}^{\circ}\)C & 1.6 (419) & 1.3 (456) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: NO\({}_{\text{x}}\)/O\({}_{\text{2}}\) ratios estimated from the TPD step under He, for CZ and Ba6/CZ, after NO\({}_{\text{x}}\) storage and reduction at the temperatures selected, and the corresponding temperature (T\({}_{\text{max}}\)) of the maximum NO + O\({}_{\text{2}}\) release, in parenthesis. Figure 7: N\({}_{\text{2}}\), NH\({}_{\text{3}}\), NO, NO\({}_{\text{2}}\), O\({}_{\text{2}}\), and H\({}_{\text{2}}\)O outlet concentrations during final TPD step, under He (after NO\({}_{\text{x}}\) storage and H\({}_{\text{2}}\) reduction 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). For the rest of catalysts, different behaviors can be seen. For Cu2/CZ, all the ad-NOx species stored are cleaned during the H2 step. In the case of Ba11/Cu2 CZ, only minor amounts of NO are desorbed. Ba6/Cu2 CZ is characterized by an intermediate behavior. NO2 evolution is not detected, but at the highest temperatures (\(\sim\)450 \({}^{\circ}\)C, a double concomitant contribution NO/O2 appears, similar to CZ). Since these peaks are shifted to higher temperatures (compared with those of CZ), this suggests that a lower amount of NOx remains on the catalyst surface at the end of the cycling process, and that these species are more stable [19; 34; 56]; therefore, higher temperatures are required for these ad-NOx species to suffer complete decomposition. When the TPD is performed after cycling at 300 \({}^{\circ}\)C, the decomposition of the remaining stored NOx produces only one temperature interval of desorption. For the NO  
Figure 8: N2, NH3, NO, NO2, O2 and H2O outlet concentrations during final TPD step, under He (after NOx storage and H2 reduction at 300 \({}^{\circ}\)C), for the catalysts: (**a**) CZ; (**b**) Cu2/CZ; (**c**) Ba6/CZ; (**d**) Ba6/Cu2 CZ; and (**e**) Ba11/Cu2 CZ. peak (the smallest one) and the NO/O\({}_{2}\) peaks, both in the case of CZ (Figure 8a) and Ba6/CZ (Figure 8c), the maximum is observed at 426 \({}^{\circ}\)C and 445 \({}^{\circ}\)C, respectively. This is in agreement with the idea explained above that the ad-NO\({}_{\rm x}\) species related to barium sites are more stable.