my answer is :
Because of the physical bonding of the Fe3O4 and MWCNTs, after each time using the nanostructure catalyst the iron oxide nanoparticles are separated from the nanostructure catalyst and the magnetic power of the nanostructure catalyst is reduced.
This is the problem usually happens in working with Fe3O4. This oxide is a mixed oxide,FeO, Fe2O3. The Fe(II) of this spinel oxide can be oxidized to Fe(III) in water and air atmosphere and then you will have less Fe3O4. The conversion of Fe(II) to Fe(III) will reduce the magnetic property of the initial oxide.
It is possible that the Fe3O4 nanoparticles of Fe3O4/MWCNT are removed from the surface and converted into superparamagnetic nanoparticles. Their ferromagnetic properties are reached from the blocking temperature, which is significantly lower than room temperature.
there may be two reasons behind. the interaction between magnetite and MWCNT, and stability of Magnetite, after recycling
Dear Maryam
I have no idea about your condition tests, temperature in particular, because in high temperature, above 500 degree C , magnetic nanoparticle materials such as Fe3O4 can be converted to paramagnetic material. ( I mean above a critical temperature or Curie point for magnetite, the material undergoes a ferrimagnetic to paramagnetic transition.
Regards
This is the problem usually happens in working with Fe3O4. This oxide is a mixed oxide,FeO, Fe2O3. The Fe(II) of this spinel oxide can be oxidized to Fe(III) in water and air atmosphere and then you will have less Fe3O4. The conversion of Fe(II) to Fe(III) will reduce the magnetic property of the initial oxide.
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