Unraveling the Role of Dopant Speciation in Oxygen-Vacancy Location and Lattice-Oxygen Supply over Fe-, Co- and Ni-Modified CeO2 NH3-SCR Catalysts

Abstract

Ceria is a promising vanadium-free catalyst for the low-temperature selective catalytic reduction of NO with NH₃ (NH₃-SCR). To establish an experimental baseline for transition-metal modification, pristine CeO₂ and Fe-, Co- and Ni-modified CeO₂ were prepared and evaluated under an identical protocol. XRD, HRTEM, Raman and XPS resolve three dopant states: Fe dissolves into the lattice and creates bulk vacancies; Ni precipitates as NiO and concentrates vacancies at the NiO/CeO₂ interface; Co phase-separates into strongly oxidising Co₃O₄. Under an oxygen-rich feed (0.5% NO, 0.5% NH₃, 5% O₂), pristine CeO₂ peaks at only 14.83% NO_x conversion at 300 °C and falls to 6.12% at 350 °C as the NO₂ yield rises to 24.92%. FeCe instead reaches 79.8% at 300 °C and retains 75.7% at 350 °C with 8.0% NO₂, demonstrating simultaneous enhancement of turnover and over-oxidation resistance. NiCe shifts the maximum to 250 °C (54.0%) but collapses to 0.3% at 350 °C, whereas CoCe produces only a narrow low-temperature window. Notably, NiCe shows the strongest defect signatures among the three modified samples by both conventional proxies, yet performs worse than FeCe above 250 °C. DFT shows that the lattice-substituted Fe site combines the lowest vacancy formation energy with the widest calculated free-energy separation between selective N–N coupling and non-selective lattice-oxygen dehydrogenation. Oxygen-vacancy location, rather than concentration alone, thus governs the activity–selectivity trade-off.

Graphical Abstract