Élaboration et étude des propriétés structurales et photocatalytiques d’oxydes mixtes dérivés de pérovskites d’orthoferrite de Lanthane (LFO) : La1-xAxFe1-yByO3

Abstract

This thesis aims to investigate the impact of cationic substitutions within a lanthanum ferrite perovskite LaFeO₃ on its physicochemical properties. By partially substituting Lanthanum with rare earth elements (Nd, Sm, Gd, Y) and Iron with Chromium (Cr), we synthesized a series of novel catalytic compounds with the formula La₁₋ₓAₓFe₁₋ᵧByO₃ (A= Gd³⁺; Nd³⁺; Y³⁺ and Sm³⁺; B= Cr³⁺) with x = 0.25 and y = 0.3. We sought to improve the structural, morphological, and optical properties of this material with the goal of optimizing its photocatalytic activity. The various catalysts were prepared using the ceramic method and then calcined at 1200°C. Their characterization was carried out using various techniques: X ray diffraction (XRD) and Rietveld's refinement, Fourier Transform Infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDS), nitrogen adsorption-desorption (BET), and UV-visible spectroscopy. Structural analyses (XRD, Rietveld refinement, FTIR, Raman, EDS) confirmed the formation of an orthorhombic perovskite structure with a space group Pnma for all compositions, indicating the successful cationic substitutions at the A and/or B sites. The examination of the morphology revealed grains of varying sizes with a pseudo-spherical shape and pores. Nitrogen adsorption-desorption isotherms showed a type II+IV behavior with a type H3 hysteresis, characteristic of mesoporous materials. The BET specific surface areas, generally low, reached a maximum value of 92.71 m²/g for the LGFCO catalyst. UV visible absorption measurements allowed the determination of the energy bandgaps (Eg) of the different materials. These values are systematically lower than that of undoped lanthanum ferrite (2.50 eV), with a minimum of 2.21 eV for the LGFCO catalyst. Photocatalytic tests, performed on the degradation of methylene blue under solar irradiation, showed a maximum degradation efficiency of 67% after 240 minutes. The reaction kinetics follows a pseudo-first order model, with a maximum apparent rate constant (kapp) of 0.00466 min⁻¹ for the LGFCO catalyst, demonstrating superior photocatalytic activity. These results suggest that this material could be a promising candidate for the degradation of organic pollutants in aqueous solution and open up interesting perspectives in the development of new high-performance photocatalytic materials for water purifyication.