Cobalt ferrites CoxFe3-xO4 (x = 1 and x = 1.5) as photocatalysts under simulated sunlight: An experimental study coupled to predictive RSM approach
Résumé
This study explores the use of CoxFe3-xO4 ferrites as photocatalyst for the degradation of a model pollutant (Rhodamine B). Two different compositions (x = 1 and 1.5) were synthesized by a hydrothermal route using nitrates as precursors. The structural studies showed that the powders consist of nanometric (8–9 nm) monocrystalline particles of octahedral shape. X-rays and electron diffraction results confirmed the cubic spinel structure. The n-type semiconductor behavior was confirmed using Mott Schottky method and the direct band gap energies were obtained from diffuse reflectance spectroscopy (DRS): 2.32 eV and 2.18 eV for CoFe2O4 and Co1.5Fe1.5O4 respectively. Due to its lower charge transfer resistance (2.8 kΩ/cm2), higher photocurrent density (1.18 µA/cm2), and a longer lifetime (5.2 ns) of the photogenerated charges, the x = 1.5 composition presents the greatest potential for this application. The response surface methodology (RSM) based on Box-Behnken design (BBD) was used to optimize the rhodamine B photodegradation process under simulated sunlight: RhB concentration, pH and catalyst mass. Under optimal conditions, the dye was completely degraded in 180 min for x = 1.5 while it was 240 min for x = 1. Total organic carbon (TOC) analysis shows an advanced mineralization and achieves a higher removal rate of 96 % for x = 1.5 after 180 min. Hydroxyl radicals and photogenerated holes are detected as the dominant reactive species using radical scavenging experiments. An increase of cobalt concentration in ferrite-based catalysts improves photocatalytic efficiency by reducing bandgap energy and improving charge photogeneration and transfer processes.
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