Reductive degradation of CCl₄ by sulfidized Fe and Pd-Fe nanoparticles: Kinetics, longevity, and morphology aspects

In this study a systematic comparison in morphology, long-term degradation, regeneration and reuse were conducted between palladized and sulfidized nanoscale zero-valent iron (Pd-Fe and S-Fe). Pd-Fe and S-Fe were prepared, after the synthesis of precursor Fe⁰ nanoparticles (spherical, ~35 nm radius) for carbon tetrachloride (CTC) treatment. With HAADF-TEM-EDS characterization, dispersive Pd islets were found on the Fe core of Pd-Fe. However, the Fe core was covered by the FeS_x shell of S-Fe (FeS/FeS₂ = 0.47). With an excessive Pd dose (10 mol%), the Pd-Fe were dramatically deformed to dendritic structures which significantly decreased reactivity. For CTC degradation, Pd-Fe (0.3 atomic% Pd) increased the degradation rate by 20-fold (k_sa = 0.580 Lm⁻² min⁻¹) while S-Fe presented a greater life time. The major intermediate chloroform (CF) was further degraded and less than 5% CF was observed after 24 h using Pd-Fe or S-Fe while above 50% CF remained using Fe. During aging, the Fe core was converted to FeOOH and Fe₃O₄/γ-Fe₂O₃. The restoration of Fe⁰ was achieved using NaBH₄, which regenerated Fe and Pd-Fe. However, the formed FeS_x shell on S-Fe was disappeared. The results suggest that S-Fe extends longevity of Fe, but the loss of FeS_x after aging makes S-Fe eventually perform like Fe in terms of CTC degradation.