Magnetizing of nano-materials on example of Degussa's P-25 TiO2 photocatalyst: Synthesis of magnetic aggregates, characterization and possible use

摘要

This study deals with the production of a magnetic photocatalyst using a new magnetizing technique - growth of magnetite crystals onto P-25 TiO2 particles from Degussa. Magnetite (MT) crystals were grown via the synthesis of layered Fe~(II)-Fe~(III) hydroxide sulfate (green rust) on the surfaces of TiO2. The magnetite/P-25 TiO2 aggregates (MT/P-25) were characterized with XRD and TEM. The structure of the P-25 TiO2 photocatalyst appeared to be unchanged by the treatment. The photocatalytic activity of MT/P-25 in aqueous suspensions were tested in the near neutral pH area where magnetite is stable and compared to the photoactivity of pure P-25 TiO2. Photooxidation of tartronic acid (TA) was first used to test the photooxidative efficiency of MT/P-25 followed by other more complex compounds: aqueous solution of 2,6 difluorobenzoic acid (2,6 DFBA) as well as the natural organic matter in water from the Hohloh Lake, Germany (NOM-HLW). For the TA photocatalytic oxidation with MT/P-25 similar oxidation rates were observed as for the P-25 TiO2, but for 2,6 DFBA and NOM-HLW the degradation rates were higher for pure P-25 TiO2. Two types of laboratory reactors were applied for photocatalytic degradation of 2,6 DFBA and NOM-HLW: a reactor (400 mL) with an artificial UV radiation source and the other one (800 mL) that was irradiated by natural solar radiation. Photonic efficiencies of organic substrates degradation processes for both types of the reactors were calculated and compared. The bigger reactor showed better results in degradation of organic substrates (photonic efficiency was more than 7 times higher for 2,6 DFBA). Magnetic separation using a simple separator (hand magnet, batch experiment) showed a mass loss less than 3% of MT/P-25 per experiment. However, using a high gradient magnetic separator (with continuous flow) less than 0.3% of MT/P-25 aggregates were lost with a suspension flow rate of 250 mL/min. In both cases magnetically separated and reused MT/P-25 showed practically unchanged photoactivity. The mechanical stability of the MT/P-25 was checked comparing the photoactivity of MT/P-25 in TA solution without prior stirring and after stirring in the reactor "in the dark" for durations of 80 and 160 h. No significant differences were observed in their photoactivity before and after stirring. However, ultrasonication of the MT/P-25 at 35 kHz for 60 min separated the MT/P-25 material into MT and TiO2. Nevertheless, for practical purposes the stability of the aggregates during the mechanical stirring and during the magnetic separation is of major importance.