The role of cobalt in bimetallic iron-cobalt magnetic carbon xerogels developed for catalytic wet peroxide oxidation

Abstract

Three magnetic carbon xerogels were developed by inclusion of iron and/or cobalt precursors during the synthesis procedure. The synthesized materials were tested in the catalytic wet peroxide oxidation (CWPO) of aqueous solutions containing 4-nitrophenol (4-NP) - a refractory organic model pollutant, under a water treatment process intensification approach. For that purpose, the experimental runs were performed with high pollutant load (5 g L-1), low catalyst dosage (2.5 g L-1, corresponding to a fixed pollutant/catalyst mass ratio of 2), atmospheric pressure, 50 degrees C, pH = 3 and stoichiometric amount of hydrogen peroxide (H2O2). The bimetallic magnetic carbon xerogel catalyst (CX/CoFe) was more active than each of the monometallic catalysts (CX/Fe or CX/Co). The better performance was explained in terms of a synergic association of factors: (i) the enhanced accessibility to the active iron species at the surface of CX/CoFe promoted by the simultaneous incorporation of cobalt, (ii) the ability of metallic Co to catalyse H2O2 decomposition via hydroxyl radicals (HO center dot) formation, and (iii) the efficient reduction of Fe3+ to Fe2+ promoted by metallic Co on the surface of CX/CoFe. A 4-NP conversion of 98.5% was determined after 30 min of CWPO reaction. Leaching of the iron species in the bimetallic CX/CoFe was considerably reduced with relation to the monometallic iron catalyst. However, partial catalyst deactivation occurred due to lower stability of oxidized cobalt species. A detailed reaction mechanism is proposed for the surface catalytic reactions occurring over the CX/CoFe catalyst.