UiO-66(Zr)-2OH-Supported Pd⁰ NP Catalysts Accelerated a Fenton-Like Reaction: Iron Cycling and Hydrogen Peroxide...

Both the sluggish kinetics of Fe(II) regeneration and usage restriction of H₂O₂ have severely hindered the scientific progress of the Fenton reaction toward practical applications. Herein, a reduction strategy of activated hydrogen, which was used to simultaneously generate H₂O₂ and accelerate the regeneration of ferrous in a Fenton-like reaction based on the reduction of activated hydrogen derived from H₂, was proposed. Two types of composite catalysts, namely, Pd/UiO-66(Zr)-2OH and Pd@UiO-66(Zr)-2OH, were successfully prepared by loading nano-Pd particles onto the outer and inner pores of UiO-66(Zr)-2OH in different loading modes, respectively. They were used to enhance the reduction of activated hydrogen. The characterization results based on the analysis of scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that the materials were successfully prepared. By using a trace amount of ferrous iron and without adding H₂O₂, trimethoprim (C₀ = 20 mg·L^-1), as a target pollutant, could be nearly 100% degraded within 180 min in the reaction system composed of these two materials. The cycle of iron and the self-generation of H₂O₂ were verified by the detection of ferrous H₂O₂ in the system. Density functional theory calculation results further confirmed that the pore-filled Pd⁰ NPs, as the main catalytic site for Pd@UiO-66(Zr)-2OH, could produce H₂O₂ under the combined action of hydrogen and oxygen. The Pd@UiO-66(Zr)-2OH system had excellent stability after multiple applications (at least 6 cycles), all of which resulted in 100% removal of trimethoprim. The degradation efficiency of the Pd/UiO-66(Zr)-2OH system for TMP gradually decreased from 97 to 80% after six cycles. The results of electron paramagnetic resonance combined with classical radical burst experiments revealed the degradation pathways in the reaction system with hydroxyl radicals and singlet oxygen as the main reactive oxygen particles.