Host: The Japan Society of Vacuum and Surface Science
Name : Annual Meeting of the Japan Society of Vacuum and Surface Science 2024
Location : [in Japanese]
Date : October 20, 2024 - October 24, 2024
1) Introduction: Hydrogen peroxide (H2O2) has attracted much attention as an environmentally friendly oxidant as well as a promising liquid fuel. Photocatalytic process is a potentially sustainable method for H2O2 production alternative to the current anthraquinone oxidation process. However, the activity is still limited, and new materials and approaches for photocatalytic H2O2 production are highly desired. Our group demonstrated that application of metal-organic framework (MOF) materials for photocatalytic H2O2 production via oxygen reduction for the first time[1]. Thanks to the advantages of the unprecedented design flexibility of MOFs, we developed the MOF materials to improve the photocatalytic activity for H2O2 production by the linker functionalization, the missing-linker defects in MOF framework, and the utilization of a two-phase reaction system with hydrophobic MOFs[1-9].
2) Ni/MIL-125-NH2: Co-catalyst deposition is an efficient approach to improve photoctalytic H2O2 production[1,2]. Ti-based MIL-125-NH2 was synthesized using titanium isopropoxide and 2-aminoterephthalic acid and deposited with Ni nanoparticles (Ni/MiL-125-NH2). Catalysts were dispersed in an O2-saturated acetonitrile solution of benzylalcohol and irradiated with visible light (λ > 420 nm). The amount of H2O2 produced on Ni/MiL-125-NH2 was much higher than that of the pristine MIL-125-NH2. Ni nanoparticles improve the selectivity for the two-electron reduction of oxygen to H2O2 through the fast disproportionation of the superoxide radicals which are the intermediate of oxygen reduction.
3) UiO-66-NH2 with missing-linker: Defect engineering for MOFs is a promising process that can modulate their electronic structure, surface chemical properties and porosity. UiO-66-NH2 consisting Zr-oxo clusters and 2-amino-terephtalate linkers was prepared. The missing-linker terminated by acetate ligands in UiO-66-NH2 were introduced by adding acetic acid during the solvothermal synthesis. Addition of missing-linker in UiO-66-NH2 framework improve photocatalytic H2O2 production[2,5]. The improvement of photocatalytic H2O2 production is attributed to not only promotion of linker-to-cluster charge transfer but also suppression of H2O2 decomposition.
4) Two-phase reaction system with hydrophobic MOF: One critical issue of the above single-phase reaction system needs an energy-consuming separation process following the H2O2 production to give separate solutions of H2O2 and the oxidation products. To solve these problems, our group developed a two-phase reaction system, which is composed of water and BA, for photocatalytic H2O2 production utilizing hydrophobic MOFs[2,6-9]. As hydrophobic modified MOFs, the linker-alkylated hydrophobic MIL-125-R was obtained by modification of the linkers in MIL-125-NH2 with alkyl anhydride. And the cluster-alkylated hydrophobic OPA/MIL-125-NH2 was prepared by modification of the clusters in MIL-125-NH2 with octadecylphosphonic acid (OPA). These samples were dispersed in an O2-saturated two-phase system composed of water and BA, and irradiated with visible light (λ > 420 nm). This two-phase reaction system realized spontaneous separation of H2O2 formed to the aqueous phase and of the benzaldehyde formed to the BA phase. The spatial separation of H2O2 and photocatalytsts improve the yield of H2O2 by preventing the H2O2 decomposition. Furthermore, the limitation in MOF stability was overcome by utilizing a hydrophobic MOF in the two-phase reaction system because the MOF particles were separated from the low pH aqueous phase. The H2O2 production in the two-phase reaction system was dramatically enhanced by the hydrophobic modification with changing cluster alkylation OPA/MIL-125-NH2 from linker alkylation MIL-125-R[6-9].
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