Promotion of NH₃-SCR activity by sulfate-modification over mesoporous Fe doped CeO₂ catalyst: Structure and mechanism

The mesoporous Fe doped CeO₂ catalyst after modifying organic sulfate functional groups show an excellent activity with above 80% NO_x conversion in a temperature range of 250-450 °C. These organic-like sulfate groups bound to the Fe-O-Ce species leads to the strong electron interaction between Fe³⁺-O-Ce⁴⁺ species and sulfate groups, which modifies the acidity and redox properties of catalyst. The strong ability of S˭O/S-O in sulfate groups to accommodate electrons from a basic molecule is a driving force in the generation of acidic properties, and thus promotes to produce new Brønsted acid sites. The bondage of Fe-O-Ce species obviously inhibits the creation of thermostable bidentate NO₃^- species. Besides, the redox cycles between Fe³⁺ and Ce⁴⁺ are disrupted, thus inhibiting NH₃ oxidation at medium-high temperatures and resulting in the increase of NO_x conversion. Furthermore, the in situ DRIFTS results show that for the fresh samples, the coordinate NH₃ reacts not only with NO₃^- through L-H mechanism, but also with oxygen species to form NO_x. Differently for sulfated sample, the coordinate NH₃ might react with achieved NO₂ instead of the oxygen species through E-R mechanism, meanwhile the NH₄⁺ could react with the NO₃^- species through L-H mechanism.