Photocatalytic decomposition of toxic phosphine gas over halloysite nanotubes co-doped by Ni and Fe3O4 and theory calculation of its mechanism

The decomposition of toxic phosphine (PH₃) gas into elemental phosphorus is a high value-added way of resource recovery and pollution control for phosphorus containing substances derived from the discharge of domestic sewage, industrial wastewater and exhaust gas. In this paper, the photocatalysis method was firstly introduced to decompose PH₃ of 1.0 v/v%. The halloysite nanotubes co-doped by Ni and Fe₃O₄ (Fe₃O₄-Ni@HNTs-NH₂) was innovatively prepared via electroless plating-precipitation method and further used as the photocatalyst for PH₃ decomposition, and it was found that the photoinitiation by 395 nm light (L-395) significantly improved the decomposition efficiency of PH₃ with Fe₃O₄-Ni@HNTs-NH₂, which was much higher than that by 365 nm light. The results of the absorption coefficient analysis, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectra indicated that the lower bandgap of Fe₃O₄ contributed to the absorption of L-395 and formation of oxygen vacancy, thereby promoting photoelectron migration, activating and breaking PH bonds in PH₃. The reaction mechanism was studied using first principles theory calculations. It is verified that there is a synergistic catalytic effect between Ni and Fe₃O₄, and Fe₃O₄ plays a major role in the photocatalysis, while the bonding orbitals of the Ni site having lower energies exhibit a better non-photocatalytic performance than Fe₃O₄. The catalytic decomposition mechanisms of PH₃ on Fe₃O₄-Ni@HNTs-NH₂ in photocatalytic and non-photocatalytic ways have been proposed. The halloysite nanotubes co-doped by Ni and Fe₃O₄ is an efficient and environment-friendly catalyst for both abatement and utilization of industrial waste PH₃ gas.