Oxygen vacancies on nanosized ceria govern the NOxstorage capacity of NSR catalysts

Pt/BaO/CeO2 catalysts derived from CeO2 nanomaterials with shapes of rods, cubes, and particles were investigated for NOx storage/reduction. Catalytic tests were performed in a transient flow reactor system. A series of characterization techniques including XRD, TEM, XPS, EXAFS, NOx-TPD, H2-TPR and in situ DRIFTS were conducted to investigate the electrical, chemical, and structural properties. The NOx storage-reduction performance ranked by the CeO2 support was nanorods > nanoparticles > nanocubes. Amazingly, the CeO2-nanorod based NSR catalyst possessed a superior NOx storage capacity (NSC) of 913.8 μmol NOx gcat−1 at 350 °C in the absence of H2O and CO2, which almost reached the theoretical value. Even under harsh lean–rich cycling conditions (90 s vs. 6 s) and a high GHSV of 360 000 h−1, the nanorod-based catalyst also showed the best reduction efficiency, affording ~ 99% NOx conversion levels from 200 °C to 400 °C under the conditions without H2O and CO2. The morphology of ceria has significant influences on the selectivity of ammonia, and on the H2O and CO2 tolerance during the NSR process. For the first time, a close linear correlation was drawn between the NOx storage capacity and the amount of oxygen vacancies of NSR catalysts. Over the NSR catalysts, oxygen vacancies play a crucial role in anchoring Pt. Meanwhile, H2-TPR results showed that the number of active surface oxygen species trapped in oxygen vacancies was closely related to the NSC value. This suggests that the oxygen vacancies on the NSR surface govern the NOx storage capacity by creating efficient sites or channels for the formation of nitrate and its further transformation to Ba-based storage sites. These findings may be fundamental for designing ceria-based NSR catalysts with better performance.