Water-dispersible Fe 3 O 4 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 Fe 3 O 4 /Al 2 O 3 catalysts to study the size-dependent activity. We identified that the activity of the catalysts in NH 3 decomposition (driven by both thermal and dielectric barrier discharge plasma) increased with increasing Fe 3 O 4 particle size. The turnover frequencies (TOFs) were increased from 0.9 to 5.8 s −1 with an increasing Fe 3 O 4 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 x N. Our results provide new evidence for the "structure-sensitivity" of the catalytic NH 3 decomposition.

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A continuous growth phenomenon is discovered for the synthesis of iron oxide nanoparticles with nanometer-scale size control in an amphiphilic solvent. The as-prepared nanoparticles are extremely water soluble without any surface modification.