Heterogeneous activation of peroxymonosulfate by LaFeO3 for diclofenac degradation: DFT-assisted mechanistic study and degradation pathways

A perovskite oxide, LaFeO3 (LFO), was synthesized and evaluated as a heterogeneous catalyst to activate peroxymonosulfate (PMS) for the oxidative degradation of diclofenac (DCF), a non-steroidal anti-inflammatory drug. It was observed that the catalytic activity of LFO was much higher than that of Fe2O3. LFO catalyzed PMS to degrade DCF with a turnover frequency (2.02 x 10(-3) min(-1))which is 17-fold higher than that of Fe2O3. Both sulfate and hydroxyl radicals were identified during LFO-activated PMS process by electron spin resonance (ESR). Radical competitive reactions indicate sulfate radicals played a major role in DCF degradation by LFO/PMS process. The PMS decomposition can be attributed to the formation of an inner-sphere complexation between the Fe (III) sites on LFO surface and PMS. Theoretical calculations illustrated the strong interaction between PMS and Fe (III) and electron transfer from PMS to Fe (III). Hydrogen temperature-programmed reduction (H-2-TPR) indicates that the LFO perovskite oxide is capable of facilitating an easier reduction of Fe (III) to mediate a redox process. Oxygen temperature-programmed desorption (O-2-TPD) suggests much more oxygen vacancies exist in LFO than in Fe2O3. Oxygen vacancies are favorable for the formation of chemical bond between Fe (III) and PMS and the activation of PMS. In situ ATR-FTIR analysis of LFO surface during PMS decomposition implies Fe (III)-Fe(II)-Fe (III) redox cycle was believed to account for the generation of sulfate radical. The intermediates generated during DCF degradation were identified and the possible degradation pathways were advanced in LFO/PMS system.