Deep insight into the structure–activity relationship of Nb modified SnO2–CeO2 catalysts for low-temperature selective catalytic reduction of NO by NH3

Abstract

Nb modified SnO₂–CeO₂ catalysts were synthesized via a co-precipitation method and used for selective catalytic reduction of NO with NH₃ (NH₃-SCR). After modifying the niobium oxides, the NH₃-SCR activity of the SnO₂–CeO₂ catalyst is greatly promoted in the temperature range of 120–210 °C and the H₂O tolerance of the catalyst is also improved in the long term operation. It is demonstrated that there is a strong interaction among Nb, Sn and Ce cations in the Nb modified SnO₂–CeO₂ catalyst. Moreover, H₂-TPR, O₂-TPD, XPS analysis and DFT calculations prove that Nb modified SnO₂–CeO₂ catalysts could provide more positive electronic charge and active electron orbits, which give rise to the varied adsorption behavior of the NH₃ species and NO_x species. Consequently, the reaction mechanism of SnO₂–CeO₂ catalysts might change after Nb modification. In situ DRIFTS was also employed to evaluate the reactivity of NH₃ with NO_x species over SnO₂–CeO₂ catalysts and Nb modified SnO₂–CeO₂ catalysts, respectively. NH₃-SCR reaction over the SnO₂–CeO₂ catalyst follows both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism. In view of the Nb modified SnO₂–CeO₂ catalyst, its reaction mainly follows the changed E–R mechanism, where abundant active coordinated NH₃ species are preferable to be activated to amide species (NH₂) and thereafter quickly react with gaseous NO to generate N₂ and H₂O. Additionally, Nb contributed to the generation of oxygen vacancies to capture the active O₂ species, leading to the enrichment of the catalytic cycle. Accordingly, the catalytic pathway of the SnO₂–CeO₂ catalyst is greatly optimized after the modification of Nb. Finally, the structure–activity relationship of the SnO₂–CeO₂ catalyst and Nb modified SnO₂–CeO₂ catalyst is also established. Deep insight into the structure–activity relationship of the Nb modified SnO₂–CeO₂ mixed oxide catalysts paves the way for a better understanding of the mechanistic aspect of Ce-based catalysts, which is helpful for developing highly efficient Ce-based catalysts for practical applications.