## 3 Results and Discussion  
### NO\({}_{x}\) Storage/Reduction Cycles  
Figure 1 shows a typical sequence of seven lean/rich cycles performed over the fully formulated Ba6/Cu2 CZ catalyst at 300 \({}^{\circ}\)C. The concentration of all the gaseous species detected (i.e., NO, NO\({}_{2}\), NO\({}_{\rm x}\), H\({}_{2}\), H\({}_{2}\)O, N\({}_{2}\), NH\({}_{3}\), CO\({}_{2}\)) are reported. Upon NO and O\({}_{2}\) admission (at t = 0), NO concentration rapidly increases, while NO\({}_{2}\) formation is observed, with a slight delay due to the occurrence of NO oxidation by O\({}_{2}\), in line with previous results [48]. The NO\({}_{\rm x}\) adsorption process reasonably involves  
Figure 1: Profiles of product and inlet concentrations (in dashed lines) during NO\({}_{\rm x}\) storage and H\({}_{2}\) reduction consecutive pulses, carried out with Ba6/Cu2 CZ at 300 \({}^{\circ}\)C.  
both Ba and Ce sites, even though NO\({}_{\rm x}\) adsorbed on Ba or on Ce sites is reported to be indistinguishable upon surface analysis. After NO admission, the evolution of H\({}_{2}\)O is observed, accompanied in the first cycle by a small amount of CO\({}_{2}\). It is possible to correlate the latter to the barium carbonate decomposition during the NO\({}_{\rm x}\) adsorption, according to the global reaction:  
\[\text{BaCO}_{3}+2\text{ NO}+3/2\text{ O}_{2}\rightarrow\text{Ba(NO}_{3})_{2}+ \text{CO}_{2} \tag{5}\]  
After the first cycle, since CO\({}_{2}\) is absent in the gas stream and carbonates are not restored during the subsequent reduction step, the evolution of CO\({}_{2}\) is not observed anymore. On the other hand, H\({}_{2}\)O desorption could be associated with the interaction between gas phase NO\({}_{\rm x}\) and the hydroxyl groups of the ceria-based catalysts, producing ad-NO\({}_{\rm x}\) surface species, as confirmed in a previous publication [48] and according to reactions (6) and (7):  
\[\text{NO}+2\text{ Ce}^{4+}-\text{OH}^{-}\rightarrow\text{Ce}^{4+}-\text{ NO}_{2}{}^{-}+\text{Ce}^{3+}-\square+\text{H}_{2}\text{O} \tag{6}\]  
\[\text{Ce}^{4+}-\text{ NO}_{2}{}^{-}+\text{Ce}^{4+}-\text{O}^{*}\rightarrow\text{Ce}^{4+}- \text{NO}_{3}{}^{-}+\text{Ce}^{3+}-\square \tag{7}\]  
The outlet concentrations of both NO and NO\({}_{2}\) increase with time and eventually reach a steady-state level (with the exception of the first adsorption step). At the end of the adsorption phase, DRIFTS analysis reported in a previous work [48] (and on SI, as commented below) showed representative bands at 1245 and 1010 cm\({}^{-1}\), which were attributed to nitrate species stored on the catalyst surface (both on Ba and Ce sites) in different geometries and configurations. At the end of every lean phase, the gas feed is switched to rich conditions to regenerate the catalyst surface from the stored NO\({}_{\rm x}\). The H\({}_{2}\) concentration presents a relevant and complex profile; indeed, it increases with the simultaneous evolution of NO and H\({}_{2}\)O, then decreases, reaching a minimum in correspondence with a maximum in the water evolution. N\({}_{2}\) and negligible amounts of NH\({}_{3}\) are monitored as N-reduction products. Accordingly, the ad-species stored on Ba sites (presumably nitrates) are reduced, following the global stoichiometry:  
\[\text{Ba(NO}_{3})_{2}+\text{H}_{2}\rightarrow\text{NO}/\text{N}_{2}/\text{ NH}_{3}+\text{BaO}+\text{H}_{2}\text{O} \tag{8}\]  
Other reactions could take place concurrently during the rich phase:  
\[\text{BaO}+\text{H}_{2}\text{O}\rightarrow\text{Ba(OH)}_{2} \tag{9}\]  
The latter could also partially explain the evolution of water during the new subsequent storage phase; indeed, when Ba hydroxides are involved in the storage, NO\({}_{\rm x}\) will scavenge H\({}_{2}\)O and the evolution of this compound will occur. The reduction of NO\({}_{\rm x}\) stored over Ce sites are considered later on.