Oxidative and adsorptive removal of chlorophenols over Fe-, N- and S-multi-doped carbon xerogels

Abstract

A range of heteroatom-doped carbon gels was studied to identify factors determining the efficiency of oxidative and adsorptive degradation/removal of phenolic compounds from aqueous solutions. Carbon xerogels were obtained via pyrolysis of resorcinol-heterocyclic aldehyde thermosetting resins. By utilizing either nitrogen-, sulfur- or oxygen-bearing heterocycles and impregnating the resins with FeCl3, carbons doped with the selected heteroatoms (N, S, O) and Fe could be obtained. This synthetic approach allows precise control of the doped element(s) amount and type. The efficiency of the chlorophenol removal process was correlated with the texture of the obtained carbon gels (for adsorptive removal), the pH of an aqueous solution and the H2O2 concentration (for oxidative degradation). The coexistence of iron and nitrogen within porous carbons enhances their catalytic properties towards oxidative decomposition of organic compounds via H2O2 activation, while the adsorption capacity for chlorophenols is directly correlated with microporosity and specific surface area values. The extensive mesoporosity and graphitic structure of the Fe and Ncodoped carbons result in efficient oxidative decomposition of the model organic pollutants. Sulfur doping prevented Fe-assisted graphitization during pyrolysis yielding disordered and highly microporous S and Fe codoped materials. The extent of oxidative degradation of chlorophenols decreased with an increasing number of Cl atoms in the phenolic ring. This report shows that Fe and Ncodoped carbons (the so-called FeNC pyrolyzed materials) are more active heterogeneous catalysts for oxidative (chloro)phenol degradation than their nitrogen- and/or iron-free counterparts, showing the importance of simultaneous coexistence of both elements (Fe and N) in the carbon scaffold.