*2.1. Characterization of the Catalysts*

Table 1 presents the nomenclature and the basic characterization data of the catalysts: B metal content (measured by ICP-OES), BET surface area (obtained by applying the BET equation to N2 adsorption data) and XPS. As it can be observed, the ICP-OES results reveal that all the metals added during the synthesis processes are present in the catalysts. Besides, the BET surface areas of the catalysts are low (as correspond to solids with negligible porosity, as mixed oxides with perovskite structure are [11]) and they range from 5 to 13 m2/g.


**Table 1.** Nomenclature and basic characterization data.

<sup>1</sup> B/Ba+Ti+B nominal = 0.1, <sup>2</sup> Olattice/Ba+Ti+B nominal = 1.5.

Concerning the XRD results, Figure 1a shows the XRD patterns of the catalysts that reveal a perovskite like structure (the diffraction peaks observed at 2θ: 22.3◦; 31.4◦; 38.8◦; 45.2◦; 51.0◦; 56.1◦; 65.8◦; 74.9◦; for (100), (110), (111), (200), (210), (211), (220), and (310) lattice planes, correspond to the standard JCPDS for tetragonal perovskite structure: 5-626 [17]) as the major crystalline phase for all the catalysts. Based on the splitting of peak around 51◦, it seems that the perovskite structure is tetragonal for BTO\_ref and BTCuO\_2, but it changes to cubic for the other catalysts [11,12]. The magnification of the main peak of the diffractograms (31.5◦), included in Figure 1b, clearly shows a shift to a lower angle value, respect to the BTO reference which is more evident for BTMnO\_2, BTCoO\_2, and BTFeO\_2 (suggesting a modification of the perovskite structure) [11,12] than for BTCuO\_2 (as the tetragonal structure is preserved for this catalyst), even though a decrease in the peak intensity is featured. Additionally, other minority phases are also identified by XRD, that is, mainly Ba2TiO4 and BaCO3 (formed by the carbonation of segregated barium oxide during samples atmospheric exposure) but also CuO for BTCuO\_2, and only BaCO3 for BTMnO\_2, BTFeO\_2, and BTCoO\_2 catalysts. As it has been previously reported [11,12], the existence of segregated phases on a metal substituted

perovskite proves that the metal has been incorporated into the perovskite structure. Therefore, the shift of the main diffraction peaks ascribed to the perovskite structure and the segregation of minority phases identified in the XRD patterns indicate that Ti is successfully substituted by Mn, Fe, Co, and Cu in the perovskite framework. It is worth mentioning that: i) for BTCoO\_2 catalysts, other cobalt phases (BaCoO3 perovskite and Co3O4) are also identified as minority segregated phases in the XRD diffractogram, suggesting that cobalt has been introduced into the perovskite lattice in a lower extent than Cu, Fe, and Mn; and ii) Ba2TiO4, which has been suggested as an active phase for NOx storage [11,12], is only detected for BTCuO\_2 catalyst.

**Figure 1.** Catalysts characterization: (**a**) XRD patterns, (**b**) main peak magnification.

To verify the structural modifications in the perovskites suggested by XRD patterns, Raman spectroscopy was used. According to literature [18,19], only the tetragonal structure of BaTiO3 perovskite, which belongs to space group P4mm, presents first-order Raman-active modes with bands at, approximately, 180 cm<sup>−</sup>1, 265 cm−1, 305 cm−1, 520 cm−1, and 720 cm−1, corresponding to irreducible representations ((A1(LO)), (A1(TO)), (B1), (A1, E(TO)), and (A1, E(LO)), respectively. On the one hand, the Raman spectra, shown in Figure 2, confirm that BTCuO\_2 preserves the original tetragonal structure of the raw perovskite, as suggested by XRD, as it features the main bands previously indicated. In spite of this, BTCuO\_2 spectrum shows broader peaks than BTO spectrum, pointing out that copper incorporation distorts the original tetragonal structure. On the other hand, BTFeO\_2 and BTMnO\_2 catalysts show an almost flat spectrum, ascribed to perovskite cubic structure, which does not show active modes in Raman spectroscopy. Finally, some Raman peaks are identified in the BTCoO\_2 spectrum which are ascribed to the presence of minority phases such as BaCoO3 and Co3O4, also identified by XRD. This result supports that a lower degree of cobalt is incorporated into the perovskite framework of the catalyst.

The different effects of the B cations, that partially substitute Ti on the perovskite structure, seem to be related with their ionic radius. Fe, Mn, and Cu (as M2<sup>+</sup>) have ionic radii larger than Ti4<sup>+</sup> causing the distortion of the raw tetragonal perovskite structure. As Co2<sup>+</sup> presents the most similar ionic radius to Ti4<sup>+</sup>, the formation of the stable BaCoO3 perovskite is also allowed and, consequently, a lower fraction of cobalt is inserted into the BaTiO3 perovskite framework to partially replace titanium.

XPS provides valuable information about the catalysts surface composition. All the XPS spectra and contributions assignment are featured in Figure A1 and Table A1, in the Appendix A. Table 1 shows the data related to metal (Mn, Fe, Co, or Cu) distribution presented as B/Ba+Ti+B (B= Mn, Fe, Co, Cu) ratio, whilst the data related to lattice oxygen, is shown as Olattice/Ba+Ti+B (B= Mn, Fe, Co, Cu) ratio. It can be observed that for Mn, Fe, and Cu, the B/Ba+Ti+B XPS ratio is lower than the corresponding nominal value (0.1), which supports that these metals have been partially introduced into the perovskite structure. The BTCuO\_2 catalyst presents the lowest value, so, the highest percentage of metal inside the perovskite lattice, whilst for Co, a B/Ba+Ti+B ratio higher than the nominal is found due to the

presence of BaCoO3 and Co3O4 segregated phases. The Olattice/Ba+Ti+B XPS ratio (calculated from the area for O1s peak corresponding to lattice oxygen) for all catalysts is lower than the corresponding value for the BTO\_ref perovskite (2.0), evidencing the creation of oxygen vacancies in the perovskite structure to compensate the imbalance in positive charge due to the partial substitution of Ti4+. Note that the BTCuO\_2 catalyst presents the lowest Olattice/Ba+Ti+B ratio and, consequently, the largest amount of surface oxygen vacancies. This result seems to be explained considering that BTCuO\_2 catalyst presents a positive imbalance larger than the other catalysts due to the highest difference between the oxidation state of Ti4<sup>+</sup> and the Cu+<sup>2</sup> (Fe and Mn appear mainly as Fe(II) and Mn(III) but Fe(III) and Mn(IV) have been also identified by XPS). Additionally, surface oxygen vacancies are also created because Ti+<sup>4</sup> cannot achieve a higher oxidation state as other B cations (as Mn or Fe in BaMn <sup>1</sup>−<sup>x</sup> CuxO3 and BaFe1-x CuxO3 [20,21]) do. Finally, it is remarkable that BTCuO\_2 catalyst preserves the original tetragonal structure (shown by XRD and Raman results), but with a high degree of distortion, which causes the presence of a larger amount of oxygen vacancies.

**Figure 2.** Catalysts characterization: Raman spectra.

#### *2.2. Catalytic Activity*

For the analysis of the activity of the catalysts for NO to NO2 oxidation and NOx adsorption/desorption, temperature programmed reaction (TPR-NOx) experiments were carried out. These experiments also allow the selection of the optimal temperature for isothermal NOx storage experiments that have been carried out in order to determine the NOx storage capacity (NSC, which is the amount of NOx stored (in μmol) per gram of catalyst). The results obtained as explained in the Materials and Methods section are presented in Figure 3.

Figure 3a features the NOx conversion profiles for all catalysts. It has been considered that positive values of NOx% conversion indicate that NOx adsorption is taking place, while negative values correspond to a NOx desorption process. According to this, at temperatures lower than 500 ◦C, approximately, the NOx conversion profiles represent NOx adsorption profiles and, at temperatures higher than 500 ◦C, these represent NOx desorption profiles. An analysis of the NOx conversion profiles reveals that BTCoO\_2 but, mainly, BTCuO\_2 catalysts show NOx adsorption/desorption activity, this performance being consistent with the presence of Ba2TiO4 segregated phase, which has been suggested as an active phase for NOx adsorption [11,12].

**Figure 3.** (**a**) NOx conversion profiles and (**b**) NO2 generation profiles during the TPR-NOx experiments.

Before analyzing the NO2 generation profiles shown in Figure 3b, it is worth mentioning that NO2 is the main compound involved in NOx adsorption processes [8,11,12] and, for this reason, it has to be considered that the NO2 registered by the analyzers is only the evolved NO2, that is: i) below 500 ◦C, it is the fraction of NO2 generated which is not stored; and ii) above 500 ◦C, it represents the NO2 that is being desorbed. Therefore, the NO2 generation shown in Figure 3b cannot be considered as a straight representation of the total NO2 generated and, consequently, any conclusion regarding NO to NO2 oxidation activity of the catalysts must be drawn from the combination of Figure 3a,b.

Thus, all the BaTi0.8B0.2O3 perovskite catalysts increase the rate of NO2 generation percentage at low temperature as the %NO2 generated is higher than that shown by the BTO perovskite used as a reference (BTO\_ref in Figure 3a,b). However, a deeper analysis of the data reveals some significant differences in the NO2 profiles of the catalysts. Firstly, BTFeO\_2 and BTMnO\_2 catalysts feature Gaussian-shape NO2 generation profiles with maxima at around 470 ◦C. Considering that almost any significant NOx adsorption/desorption activity is observed for these catalysts, it can be suggested that they are mainly active for NO to NO2 oxidation (however, NOx adsorption capacity cannot be totally ruled out due to the intrinsic characteristics of TPR experiment). Secondly, although BTCoO\_2 catalyst presents a similar type of NO2 generation profile, it shows the highest NO oxidation activity, which seems to be related to the presence of Co3O4, as metal oxides are active for the NO to NO2 oxidation reaction [11,12,14–16]. In addition, the low intensity NOx conversion peaks observed for BTCoO\_2 catalyst in Figure 3a, indicates a low NOx adsorption activity that, according to literature [14–16], could be due to the presence of minority segregated phases and oxygen vacancies. Finally, a different NO2 generation profile with two maxima and a minimum (at ca. 421, 507, and 441 ◦C, respectively) are

clearly identified for BTCuO\_2 catalyst. It is worth indicating that the temperature of the minimum NO2 generation perfectly matches with the temperature of the maximum NOx conversion observed for this catalyst in Figure 3a. This result points out that, at this temperature, BTCuO\_2 shows higher NOx adsorption rate than NO oxidation rate as Ba2TiO4 phase, which is active for NOx adsorption [11,12], has been identified.

As a summary, TPR- NOx results reveal that only BTCuO\_2 presents the NOx conversion and NO2 generation profiles expected for LNT catalysts [11,12]. Thus, even though all the catalysts present active sites for NO-to-NO2 oxidation, such as oxygen vacancies and surface metal oxides, only the catalyst containing copper shows the presence of the Ba2TiO4 segregated phase, which is active for NOx adsorption [11,12].

In order to determine the NSC, NOx storage experiments at 400 ◦C (the minimum temperature for NOx adsorption in TPR-NOx profiles) have been carry out for the three perovskites in which Ti has been substituted in a larger degree, that is, BTCuO\_2, BTFeO\_2, and BTMnO\_2. The NSC values (shown in Table 2) have been obtained during the 10th NOx storage cycle at which the catalysts achieve a stable performance (see Materials and Methods section for more details). Figure 4 shows, as an example, the NO, NO2, and NOx profiles during NSC experiments at 400 ◦C, corresponding to the BTCuO\_2 catalyst.

**Table 2.** NSC data at 400 ◦C for BTO reference, BaTi0.8B0.2O3 catalysts and for some reference noble metal-base catalysts. q


**Figure 4.** NSC cycles at 400 ◦C for the BTCuO\_2 catalyst.

Data on Table 2 reveals that the three catalysts present a measurable NSC, but, in agreement with TPR-NOx results, BTCuO\_2 catalyst is the most active one. The characterization results previously discussed allow us to justify the high NSC shown by copper perovskite. On the one hand, the incorporation of Cu into the perovskite lattice distorts the raw tetragonal structure, generates the largest pool of oxygen vacancies, and promotes the segregation of mainly Ba2TiO4 and BaCO3, but also CuO as segregated phases that seem to be the active sites for both NO to NO2 oxidation and NOx storage [11,12]. On the other hand, the insertion of Mn and Fe causes a structural change from

tetragonal to cubic, but only BaCO3 appears as segregated phase and a lower amount of oxygen vacancies respect to Cu. Consequently, a lower NO to NO2 oxidation activity and NSC is shown by these two catalysts. Finally, it is important to underline that the NSC of BTCuO\_2 is within the range of values reported for noble metal/alkali or alkali earth base catalysts (Table 2). Moreover, the BTCuO\_2 perovskite does not incorporate any noble metal, and therefore it could be a cheaper alternative to current catalysts based on noble metals. Additionally, as this catalyst works at 400 ◦C, presenting an acceptable NOx storage capacity, it could be proposed as a component of high-temperature LNT for lean burn gasoline engines (GDI gasoline direct injection) which need catalysts working between 400 and 500 ◦C.
