Selective catalytic oxidation of ammonia by efficient FeOx in coal ash: insights from experimental and DFT studies

Ammonia-coal co-firing is a promising strategy for reducing CO 2 emissions while utilizing existing coal infrastructure. However, challenges remain in controlling nitrogen oxide (NO x ) emissions and unreacted ammonia (NH 3 ). This study investigates the role of iron oxide (FeO x ) species in coal ash for the selective catalytic oxidation (SCO) of NH 3 during co-firing, combining experimental and density functional theory (DFT) approaches. Four FeO x species (α-Fe 2 O 3 , γ-Fe 2 O 3 , Fe 3 O 4 , FeO) were systematically evaluated for their NH 3 -SCO performance over the temperature range of 300–1000°C under varying oxygen concentrations. Experimental results showed a distinct catalytic activity trend: α-Fe 2 O 3 > γ-Fe 2 O 3 > Fe 3 O 4 > FeO, with Fe 3 O 4 exhibiting superior N 2 selectivity due to its abundant surface oxygen species and reduced oxidation state. Oxygen concentration was found to significantly affect reaction pathways; lower O 2 levels enhanced N 2 selectivity for Fe 3 O 4 and FeO, while α-Fe 2 O 3 and γ-Fe 2 O 3 maintained stable oxidation performance. DFT calculations confirmed experimental observations, revealing NH 3 adsorption energies (−1.79 to −0.91 eV) and identifying the *NH 3 → *NH 2 step as the rate-determining step. Mechanistic analysis highlighted an internal selective catalytic reduction (i-SCR) pathway, where energy barriers dictated product selectivity: α-Fe 2 O 3 /γ-Fe 2 O 3 favored NO formation, whereas Fe 3 O 4 /FeO promoted N 2 generation. This study provides valuable insights into the catalytic behavior of different FeO x species in coal ash, offering a foundation for optimizing ammonia-coal co-firing systems to mitigate both NO x and NH 3 emissions synergistically.