Facile Fabrication of Uniform MFe₂O₄ (M = Co, Ni, Cu) Hollow Spheres and Their Recyclable Superior Catalytic Activity Towards 4-Nitrophenol Reduction.

We have developed a facile, efficient, economic, and green method to fabricate uniform MFe2O4 (M Co, Ni, Cu) hollow spheres by using polyacrylamide (PAM) as an additive agent. The as-obtained three types of hollow ferrites are employed to catalyze the reductive transformation of nitrophenols to aminophenols, which exhibit excellent catalytic activity. The kinetics of the catalytic reaction follows the quasi-first order reaction and its rate constant (kapp) values for CoFe2O4, NiFe2O4 and CuFe2O4 are calculated to be 1.62, 11.74 and 37.28 min-1, respectively. These data are higher than those former reports which using similar reactions catalyzed by different nanocatalysts. Among the three catalysts, NiFe2O4 and CuFe2O4 can be successfully reused for at least six consecutive cycles and have a stable conversion efficiency of about 100%, while CoFe2O4 is catalytically inactive after three cycles. The superiority of the catalytic reduction of NiFe2O4 and CuFe2O4 may be due to their soft magnetic properties as well as their better separation efficiency of the electron-hole pairs. Thus it is expected that these two recyclable hollow ferrites will be used commercially in the treatment of aromatic nitro compounds in industrial wastewater.