## 3 Results and Discussion
### Effect of Catalyst Formulation on the Reduction Phase
On the other hand, a part of these consumptions is attributed to nitrite/nitrate reduction and can be associated with the NO\({}_{\rm x}\) stored in the previous step, that being the copper-containing catalysts with higher consumptions. H\({}_{2}\) consumption does not increase linearly with temperature due to the creation of different populations of ad-NO\({}_{\rm x}\) species at different temperatures as a function of this variable. Figure 5: H\({}_{2}\) consumption in the reduction step (2000 ppm H\({}_{2}\) for 1 h) at several temperatures. A detailed inspection of Figure 5 reveals that there is a relative maximum of H\({}_{2}\) consumption at 250 \({}^{\circ}\)C in most of the samples. First of all, copper-containing samples achieve their maximum H\({}_{2}\) consumption at approximately 250 \({}^{\circ}\)C (in agreement with the H\({}_{2}\)-TPR analyses), which should be accounted for in explaining the H\({}_{2}\) consumption trends observed. Therefore, a very low H\({}_{2}\) consumption is found at 150 \({}^{\circ}\)C; when the temperature is increased up to 250 \({}^{\circ}\)C or more, the catalysts are easily reduced. Cu-containing catalysts show different trends over 250 \({}^{\circ}\)C, which are directly related to the NOx adsorption capacity. Cu2/CZ shows a decrease in NOx adsorption capacity together with temperature (congruent, with a corresponding decrease in H\({}_{2}\) consumption), while Ba/Cu-based catalysts present different trends when temperature goes up to 300 \({}^{\circ}\)C and 350 \({}^{\circ}\)C, respectively, with higher amounts of NOx stored in the previous step. Ba11/Cu2 CZ shows a lower H\({}_{2}\) consumption with respect to Ba6/Cu2 CZ due to the slightly lower amount of Cu present when the Ba loading increases. On the other hand, copper-free samples show very low H\({}_{2}\) uptakes, becoming slightly higher when the temperature increases, which might be ascribed to the marginally enhanced reducibility of ceria-zirconia upon temperature increase, and also to the different nitrite/nitrate population creation on the Ba6/CZ and CZ with temperature. In summary, a balance between the NOx adsorption capacity and the reduction extension, at whatever temperature, should be considered when analyzing H\({}_{2}\) consumption amounts in terms of temperature. The evolution of surface species during the reduction phase was studied by in situ DRIFT analysis. Since the focus of this work is the reduction phase, the spectra recorded during the storage step, which represent the starting point of this analysis, are reported in Figures S3 and S4 in the Supplementary Information for the CZ and Ba6/CZ catalysts, respectively. Note that this characterization is possible for these two catalysts, while Cu is present, the dark-green color of the resultant samples leads to very low-intensity DRIFTS signals, making the DRIFT analysis very complex. However, with Ce and Ba as the adsorption sites, it is supposed that the spectra of the fully formulated Ba/Cu catalysts closely resemble those recorded for CZ and Ba6/CZ with respect to the nature of the adsorbed species, while Cu has more influence on the reduction dynamics. Figure 6a,b presents the DRIFTS spectra during the H\({}_{2}\) reduction step for CZ and Ba6/CZ, respectively, at 300 \({}^{\circ}\)C. For CZ, the band associated with adsorbed nitrate species (1279 cm\({}^{-1}\)) [54; 55], decreases in intensity during the reduction and completely disappears after 20 min; the same behavior is observed for the associate band at around 1025 cm\({}^{-1}\). In the region between 1600 and 1200 cm\({}^{-1}\), residual broad bands probably related to residual carbonates (e.g., 1482 cm\({}^{-1}\)) are observed up to the end of the reduction phase. Regarding the adsorbed hydroxyl species, small changes can be observed during the reduction, although a slight increase in the band at 3637 cm\({}^{-1}\) could be observed, since the desorption of water during this step could make the OH groups visible.