A composite photocatalyst of an organic electron donor-acceptor dyad and a Pt catalyst supported on semiconductor nanosheets for efficient hydrogen evolution from oxalic acid

A composite photocatalytic system for hydrogen evolution employing acidic oxalic acid as an electron donor has been successfully constructed by combining 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh⁺-NA), platinum (Pt) and nanosheets prepared by the exfoliation of K₄Nb₆O₁₇ (niobate-NS) as an organic photosensitiser, a hydrogen-evolution catalyst and a semiconductor photocatalyst for the oxidation of oxalic acid, respectively. The composite photocatalyst, QuPh⁺-NA/niobate-NS (Pt), was prepared by a two-step route to locate a Pt catalyst near QuPh⁺-NA on the surface of niobate-NS: (i) supporting QuPh⁺-NA on niobate-NS by a cation exchange method and then (ii) supporting Pt on the QuPh⁺-NA/niobate-NS by a photodeposition method using PtCl₄^2- as a precursor, which interacts repulsively with the negatively charged surface of niobate-NS. The precursor of PtCl₄^2- was reduced to metallic Pt by the photocatalysis of QuPh⁺-NA in the presence of oxalate. Photocatalytic hydrogen evolution with the composite catalyst proceeds via photoexcitation of both niobate-NS and QuPh⁺-NA to produce an electron and a hole in the semiconductor and the ET state (QuPh·-NA·⁺), respectively. The photogenerated hole of niobate-NS oxidises oxalic acid to produce CO₂ and CO₂·^- with two protons, whereas the photogenerated electron and CO₂·^- reduce QuPh⁺-NA and the electron-transfer (ET) state to produce two equivalents of QuPh·-NA, which inject electrons to the Pt catalyst to reduce protons to hydrogen. The utilisation of oxalic acid as an electron donor even under highly acidic conditions, which are thermodynamically favourable for proton reduction to evolve hydrogen but unfavourable for oxalate oxidation, has been made possible for the first time by combining QuPh⁺-NA, Pt and niobate-NS. Composite photocatalysts were also prepared by employing mesoporous silica-alumina and nanosheets prepared by the exfoliation of KTiNbO₅ (titanoniobate-NS), which possesses a band structure different from niobate-NS, as supports to clarify the requirements for a building block to achieve an active composite photocatalyst.