Investigations On Preparation And Structure Of Core-Shell Magnetic Photocatalyst And Its Photocatalytic Properties

Anatase TiO₂ nanoparticle presents good photocatalytic activity for effective degradation of organic waste in water. However, it is not easily separated from wasting water due to its small particles. Magnetic photocatalyst is a novel catalyst with magnetic material core. It not only possesses favorable photocatalytic activity, but also can be expediently separated by an external magnetic field, so it can solve the problem of catalyst separation. But the magnetic photocatalysts most recently repoted suffer from the disadvantage involving Fe₃O₄ core easily oxidized and surface OH groups decreased after high temperature treatment.In present paper, a series of novel core-shell structured magnetic photocatalyst TiO₂/MFe₂O₄ and TiO₂/SiO₂/MFe₂O₄ (M=Co, Mg) was firstly prepared by urea hydrolysis process involving anatase TiO₂ nanoparticles directly hydrolyzed from aqueous soluble TALH and coated on spinel ferrite cores. The core-shell structure of the prepared photocatalyst was validated. The physicochemical properties and photoactivity of the magnetic photocatalyst were also studied. The main conclusions were presented below:(1) The heat-treatment temperature is a key factor in the preparation of anatase TiO₂ directly hydrolyzed from TALH by urea hydrolysis method. The surface adsorbent water molecules, surface OH groups and organic residues were gradually lost and the crystallinity degree of anatase TiO₂ becomes better with increasing temperature. The sample calcined at 300℃present the highest photoactivity for the degradation of methyl orange, and this can be attributed to the well-ordered anatase crystal structure and proper amount of OH groups for capturing photogenerated electrons and holes.(2) A series of novel core-shell structure magnetic photocatalyst TiO₂/MFe₂O₄ and TiO₂/SiO₂/MFe₂O₄ (M=Co, Mg) was prepared with magneitic spinel ferrite MFe₂O₄ (M=Co, Mg) as core and TALH as tiantia precursor by urea hydrolysis method.Firstly, the effect of inert SiO₂ medial laryer on the photoactivity of magnetic photocatlyst was studied in both TiO₂/SiO₂/MgFe₂O₄ and TiO₂/SiO₂/CoFe₂O₄ systems, compared with that of TiO₂/MFe₂O₄ (M=Co, Mg) samples. The results show that wide-bandgap SiO₂ medial laryer prevent the photogenerated electrons and holes on TiO₂ from transferring into narrow-bandgap MFe₂O₄ (M=Co, Mg). So the transfer and recombination between the electrons and holes were effectively restrained resulting in better photoactivity of magnetic photocatalyst.Sencondly, the core-shell structure and the photoactivtity of magnetic photocatlyst TiO₂/SiO₂/CoFe₂O₄ were studied. The core (CoFe₂O₄)-inert medial layer (SiO₂)-shell (TiO₂) structure of TiO₂/SiO₂/CoFe₂O₄ was confirmed by using TEM, SEM-EDS and XPS techniques. TiO₂ shell was connected with SiO₂ inert medial layer through Si-O-Ti linkage. The sketch of core-shell structure of TiO₂/SiO₂/CoFe₂O₄ was proposed based upon the integrated characterization resutls. The photoactivity of TiO₂/SiO₂/CoFe₂O₄ is similar to that of TiO₂ obtanied from the same precursor and Degussa P25, this is attributed to the well-ordered anantase phase of TiO₂ shell and the shielding function of the SiO₂ medial laryer.Lastly, magnetic separation ratio and the structure of TiO₂/SiO₂/CoFe₂O₄ after first run were also done. The photoactivity of the magnetic reclaimed catalyst after three runs was also examined. The results illustrate the constant numbers of characteristic reflection peaks and sublte change of absorption intensity of Ti-OH groups. After 250 min irradiation, the photodegradation of methyl orange in three runs are 29.8%,16.4% and 12.8%, respectively. The photoactivity in first time is the highest. The decrease of photoactivity probably results from congregation of the catalyst particles in external magnetic field.(3) The kinetics of photodegradation of methyl orange over the prepared photocatalysts was found to follow a pseudo-first-order rate law.