## 4 Conclusions  
This research was dedicated to the study of new PGM-free catalysts for the NO\({}_{\rm x}\) removal during LNT operation under simulated conditions. For this purpose, Ba-Cu/ceria--zirconia catalysts were synthetized, characterized, and tested by means of lean/rich cycles under isothermal conditions, at low temperatures, with particular attention to the reduction step in order to investigate their catalytic behavior in terms of the influence of copper presence on the catalytic formulations and the incorporation of different barium loadings to the ceria-zirconia support. For Cu-free samples (i.e., CZ and Ba6/CZ) the addition of H\({}_{2}\) has the sole effect of partially reducing the catalytic surface, while the small amounts of stored NO\({}_{\rm x}\) are thermally decomposed to NO\({}_{2}\) and/or to NO/O\({}_{2}\) during the final TPD. On the other hand, for Cu-containing catalysts, N-products are detected during the rich phase, i.e., NO, N\({}_{2}\) with negligible amounts of NH\({}_{3}\), and N\({}_{2}\)O. A mechanism for the stored NO\({}_{\rm x}\) reduction has been proposed, which involves the formation of oxygen vacancies over the Ce sites responsible for the reduction of released NO\({}_{\rm x}\) to N\({}_{2}\) and N\({}_{2}\)O. These results are promising since they give us the idea that Cu/ceria-based catalysts can produce N\({}_{2}\) even in the absence of noble metals. Further investigations on this aspect can be carried out in the future, focusing on the optimization of ceria-based materials for efficient NO\({}_{\rm x}\) storage and reduction processes and yielding N\({}_{2}\) as the main product,since very few investigations have been published in this area using these kinds of materials and obtaining such detailed information in terms of the nature of evolved products, the selectivity towards the desired products, and mass balances, as far as the authors are concerned. However, it is worth considering that other different reaction pathways might occur, involving the generation of nitritites and nitrates on the catalysts' surfaces. In fact, other related reactions' mechanisms have been published in the literature to explain the NOx interactions with cerium sites and oxygen vacancies, yielding the formation of mostly nitrites, hyponitrites, and nitrates. The following are available online at [[https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)]([https://www.mdpi.com/article/10](https://www.mdpi.com/article/10)) _.3390/app11125700/s1. Figure S1: NO, NO\({}_{2}\), NOx,CO\({}_{2}\) and H\({}_{2}\)O outlet concentrations, and NO inlet profile (in dotted lines) of NOx adsorption step, under 1000 ppm NO + 3% O\({}_{2}\) (\(v/v\)), at 150 \({}^{\circ}\)C, for the catalysts, Figure S2: NO, NO\({}_{2}\), NOx,CO\({}_{2}\) and H\({}_{2}\)O outlet concentrations, and NO inlet profile (in dotted lines) of NOx adsorption step, under 1000 ppm NO + 3% O\({}_{2}\) (\(v/v\)), at 300 \({}^{\circ}\)C, for the catalysts, Figure S3: In situ DRIFT spectra of CZ during NOx adsorption step, at 300 \({}^{\circ}\)C, Figure S4: In situ DRIFT spectra of Ba6/CZ during NOx adsorption step, at 300 \({}^{\circ}\)C, Table S1: NOx adsorbed amounts obtained from the experiments illustrated on Figures S1 and S2. Conceptualization, L.C. and A.G.-G.; methodology, [PERSON]-M., [PERSON], R.M., L.C. and A.G.-G.; validation, L.C. and A.G.-G.; formal analysis, [PERSON] and [PERSON]; investigation, [PERSON] and [PERSON]; data curation, [PERSON]-M., [PERSON] and R.M. ; writing--original draft preparation, [PERSON], L.C. and [PERSON]-G.; writing--review and editing, [PERSON], R.M., L.C. and [PERSON]; supervision, [PERSON]; funding acquisition, [PERSON]. All authors have read and agreed to the published version of the manuscript. This research was funded by the financial support from the Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542 RB-I00 project) and the UE-FEDER funding.