Experimental and theoretical study on the reactivity of maghemite doped with Cu2+ in oxidation reactions: structural and thermodynamic properties towards a Fenton catalyst

Abstract

In this work, a polymeric method was used to prepare undoped and Cu-doped iron oxide catalysts for the H₂O₂ decomposition reaction. These catalysts were characterized by powder X-ray diffractometry (XRD), scanning electronic microscopy (SEM) coupled to an energy dispersive X-ray spectrometer (EDX), and H₂-Temperature Programmed Reduction (H₂-TPR). The SEM images show an inhomogeneous particle cluster in both samples, tending to decrease in size with Cu-doping. EDX mapping reveals a good dispersion of Cu²⁺ in the iron oxide. In addition, Rietveld refinement of the XRD patterns reveals that the samples are constituted of hematite and maghemite, but only maghemite has octahedral Fe³⁺ ions isomorphically replaced by 2 wt% Cu²⁺. Cu-doping produces an active catalyst for H₂O₂ decomposition. Tests using phenol show the strong inhibition of H₂O₂ decomposition by the Cu-doped catalysts, suggesting that H₂O₂ may be decomposed via a radical mechanism. Furthermore, phenol degradation kinetics confirm that the doping of maghemite with Cu²⁺ brings about a significant improvement in catalytic activity. Theoretical calculations reveal that Cu-doping in maghemite produces low electronic density sites, favoring the interactions between the surface oxygens of H₂O₂ and Cu²⁺, thus improving the catalytic activity. This strategy can be extended to other materials to design active heterogeneous catalysts for environmental purposes.