Photocatalytic Recovery of Noble Metals by Covalent Silyl Polyoxophosphotungstate-Porphyrin Copolymers

The photocatalytic recovery of noble metals on photosensitive semiconductors such as TiO₂ is well-established for forming M/TiO₂ but has notable drawbacks. TiO₂ suffers from low conversion efficiency due to significant recombination of photogenerated electron-hole pairs. Its wide energy band gap (3.2 eV) also restricts excitation to high-energy UV light, limiting its use with solar energy. This study proposes an efficient alternative using hybrid polyoxometalate (POM)-porphyrin copolymers for the photocatalytic recovery of Ag and Pt under visible light. Copolymeric films composed of hybrid polyoxometalates and porphyrins have been obtained by the electrooxidation of 5,15-(di-p-tolyl)porphyrin (H) or 2,3,7,8,12,13,17,18-octaethylporphine zinc(II) (ZnOEP) together with Keggin or Wells-Dawson-type organosilyl polyoxophosphotungstate ([PW₁₁Si₂O₄₀C₂₆H₁₆N₂]TBA₃ or [P₂W₁₇Si₂O₆₂C₂₆H₁₆N₂]TBA₆). In these films, porphyrin subunits can be excited under visible illumination, acting as photosensitizers that transfer electrons to the polyoxometalate catalysts. Notably, POM-porphyrin films demonstrated high efficiency in Pt(IV) photoreduction over repeated cycles without catalyst degradation.