Synergistic effects of SO42− /PO43− modification and Fe-loading on NH3-SCR activity in surface-engineered rare earth concentrate catalysts

Abstract

Bayan Obo rare earth concentrate (REC), naturally rich in Ce and low in transition metals, exhibits high stability and poison resistance, making it promising for NH₃-SCR. The REC was first modified with H₂SO₄ (S) or H₃PO₄ (P). Optimizing acid concentration revealed that 6 mol/L H₂SO₄ and 4 mol/L H₃PO₄ yielded catalysts with the best SCR activity. To broaden the temperature window and enhance activity, acid-treated REC(REC-S/P) was further modified with Fe via impregnation, creating Fe₂O₃/REC-(S/P) catalysts. Studies on varying Fe/Ce ratios showed an optimal loading (Fe/Ce = 0.8) significantly expanded the operating temperature range and improved N₂ selectivity. These catalysts achieved >90 % NO_x conversion between 325 and 375 °C. Iron loading critically enhanced SCR activity by improving redox properties and increasing available acidic sites. In-situ IR studies explored adsorption (NH₃, NO, O₂) and reaction pathways. Adsorbed NO was oxidized by surface Fe³⁺ to form –NO₂^- and –NO₃^- species, primarily via the Langmuir-Hinshelwood (L-H) mechanism. At higher temperatures, adsorbed NH₃ species activated by Fe³⁺ formed amide (NH₂). Crucially, this NH₂ on Fe₂O₃/REC-(S/P) could be oxidized to N₂O, predominantly through the Eley-Rideal (E-R) mechanism.