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

Abstract

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 (H₂T₂P) 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.