Size-Dependent Activity of Iron Nanoparticles in Both Thermal and Plasma Driven Catalytic Ammonia Decomposition

Water-dispersible Fe₃O₄ nanoparticles with diameters of 4.2 ± 0.6, 6.1 ± 0.8, 8.1 ± 1, and 10.4 ± 1 nm were prepared through the polyol method and employed as the precursors of Fe₃O₄/Al₂O₃ catalysts to study the size-dependent activity. We identified that the activity of the catalysts in NH₃ decomposition (driven by both thermal and dielectric barrier discharge plasma) increased with increasing Fe₃O₄ particle size. The turnover frequencies (TOFs) were increased from 0.9 to 5.8 s^–1 with an increasing Fe₃O₄ precursor size from 4.2 to 10.4 nm during the thermocatalytic decomposition. A quite similar “particle size effect” was also observed for the plasma catalytic decomposition, although lower TOF was observed. Additionally, reaction-induced catalyst reconstruction was identified during the early-stage of the catalytic decomposition and can be attributed to the nitridation of FeO_x to Fe_xN. Our results provide new evidence for the “structure-sensitivity” of the catalytic NH₃ decomposition.