Composition–structure–activity relationships in CuxCo1−xFe2O4 spinel ferrites for visible-light photo-Fenton degradation of rhodamine B

Abstract

Although Cu-substituted CoFe2O4 ferrites have been extensively investigated as visible-light photo-Fenton catalysts, the physicochemical basis of their composition-dependent activity maximum remains insufficiently resolved. Herein, a systematic CuxCo1−xFe2O4 series (x = 0–0.5) was synthesized within a single spinel-ferrite framework, enabling crystallographic, vibrational, morphological, textural, optical, and catalytic characteristics to be evaluated consistently across the investigated composition range. The cubic spinel structure was retained throughout the series, while Cu substitution induced systematic changes in lattice characteristics, local coordination-related vibrational features, microstructure, and optical properties. These changes were most pronounced at intermediate Cu contents (x = 0.3–0.4), whereas the optical band-gap energies remained within 1.34–1.60 eV. Rhodamine B degradation increased from 76.45% for pristine CoFe2O4 to 98.78% for Cu0.4Co0.6Fe2O4, accompanied by an approximately 2.9-fold increase in the apparent reaction-rate constant. The optimized catalyst retained approximately 92% of its initial degradation efficiency after four consecutive reuse cycles. Mineralization assessment further indicated substantial, although incomplete, organic-carbon removal, with 84.56% TOC removal at 240 min and 90.8% COD removal at 360 min under the investigated conditions. Radical-scavenging experiments supported ˙OH as the predominant reactive species, while the elementary radical-generation pathways remained unresolved. A central finding is that the highest photocatalytic activity did not coincide with either the maximum BET surface area or the minimum optical band-gap energy. The activity maximum occurred alongside pronounced changes in crystallographic and local coordination-related vibrational characteristics, but these features cannot be assigned as independent causal determinants from the present dataset. Within the limits of the present dataset, the activity trend is therefore more consistently associated with the coupled evolution of composition-dependent physicochemical characteristics than with any single measured descriptor. These findings provide experimentally supported insight into the composition–structure–activity relationships observed within the investigated CuxCo1−xFe2O4 system.