Mechanism of polyhydroquinone coating iron/copper oxides for enhanced catalytic activity

In this study, Polyhydroquinone (PHQ) coated eight kinds of iron/copper oxides (CuO, Cu2O, Fe3O4, Fe2O3, C-CuFe2O4, T-CuFe2O4, 3R-CuFeO2, 2H-CuFeO2) to enhance their catalytic activity. As evidenced by the characterization results, PHQ was coated at Cu(I), Cu(II), and Fe(II) sites rather than Fe(III) sites by forming metal-organic complexes at the catalyst surface. These results were supported by catalytic experiments of Persulfate (PS) activation for 1,4-dioxane degradation. Based on the normalized pseudo-first-order kinetics constant, PHQ enhanced iron/copper sites in the order of Cu(I) > Fe(II) ≈ Cu(II). Interestingly, in contrast to the general agreement, the Hard and Soft Acids and Bases (HSAB) Principle indicated that the benzene ring (soft base) rather than the phenolic hydroxyl group (hard base) was the reactive site between Fe/Cu and PHQ. Six of the above eight iron/copper oxides (CuO, Cu2O, Fe3O4, C-CuFe2O4, 3R-CuFeO2, 2H-CuFeO2) displayed improved catalytic performance. The other two catalysts (Fe2O3 and T-CuFe2O4) showed no improvement in activity after PHQ coating. Among all the catalysts, PHQ/2H-CuFeO2 showed the best catalytic performance, which was also able to activate Hydrogen peroxide (HP) and Peroxymonosulfate (PMS) for 1,4-dioxane degradation by a radical pathway. However, radical process caused the decomposition of PHQ at the catalyst surface, leading to a dramatic drop (from 100% to approximately 30%) in catalyst activity. In contrast, PHQ/2H-CuFeO2 activated PS by a nonradical pathway where Cu(III) was likely responsible for 1,4-dioxane degradation. PHQ/2H-CuFeO2 was more stable after activating PS.