Oxygen vacancies on nanosized ceria govern the NOx storage capacity of NSR catalysts

Abstract

Pt/BaO/CeO₂ catalysts derived from CeO₂ nanomaterials with shapes of rods, cubes, and particles were investigated for NO_x storage/reduction. Catalytic tests were performed in a transient flow reactor system. A series of characterization techniques including XRD, TEM, XPS, EXAFS, NO_x-TPD, H₂-TPR and in situ DRIFTS were conducted to investigate the electrical, chemical, and structural properties. The NO_x storage-reduction performance ranked by the CeO₂ support was nanorods > nanoparticles > nanocubes. Amazingly, the CeO₂-nanorod based NSR catalyst possessed a superior NO_x storage capacity (NSC) of 913.8 μmol NO_x g_cat⁻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% NO_x 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 NO_x 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 NO_x 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.