Physicochemical characteristics of calcined MnFe2O4 solid nanospheres and their catalytic activity to oxidize para-nitrophenol with peroxymonosulfate and n-C-7 asphaltenes with air

Manganese ferrite solid nanospheres (MSNs) were prepared by a solvothermal method and calcined at various temperatures up to 500 degrees C. Their surface area, morphology, particle size, weight change during calcination, surface coordination number of metal ions, oxidation state, crystal structure, crystallite size, and magnetic properties were studied. The MSNs were used as catalysts to activate potassium peroxymonosulfate (PMS) for the oxidative degradation of para-nitrophenol (PNP) from water and for the oxidation of n-C-7 asphaltenes in flowing air at atmospheric (0.084 MPa) and high pressure (6 MPa). Mn was in oxidation states (II) and (III) at calcination temperature of 200 degrees C, and the crystalline structure corresponded to jacobsite. Mn was in oxidation states (III) and (IV) at 350 degrees C and in oxidation states (II), (III), and (IV) at 500 degrees C, and the crystalline structure was maghemite at both temperatures. MSN catalysts generated hydroxyl (HO center dot) and sulfate (SO4 center dot-) radicals in the PMS activation and generated HO center dot radicals in the n-C-7 asphaltene oxidation. In both reactions, the best catalyst was MSN calcined at 350 degrees C (MSN350), because it has the highest concentration of Mn(III) in octahedral B sites, which are directly exposed to the catalyst surface, and the largest total and lattice oxygen contents, favoring oxygen mobility for Mn redox cycles. The MSN350 sample reduces the decomposition temperature of n-C-7 asphaltenes from 430 to 210 degrees C at 0.084 MPa and from 370 to 200 degrees C at 6.0 MPa. In addition, it reduces the effective activation energy by approximately 77.6% in the second combustion (SC) region, where high-temperature oxidation reactions take place.